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/NBS building science series TA435 .U58 V46;1973 C.I NBS-PUB-C 1974 PUBLICATION

BUILDING SCIENCE SERIES 46

.U58

U.S.

DEPARTMENT

OF

COMMERCE

National

Bureau

of

Standards

Building Practices

for Disaster Mitigation

Building Practices for Disaster Mitigation

%)\\d\m 5C\e/\Ce NaW Bureau m Stanuards A MAY MAY 1 9 lOilA 1 3 1974

Proceedings of a Workshop Sponsored by ^, The National Science Foundation,

'

Reserch Applied to National Needs Program

and

The National Bureau of Standards I 1

Held at the National Bureau of Standards

Boulder, Colorado

August 28-September 1, 1972

Edited by

Richard Wright, Samuel Kramer, and

Charles Culver

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Issued February 1973
Library of Congress Catalog Card Number: 73-600006

National Bureau of Standards Building Science Series 46

Nat. Bur. Stand. (U.S.), Bldg. Sci. Sen 46, 483 pages (Feb. 1973) CODEN: BSSNBV

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

• (Order by SD Catalog No. C13.29/2:46). Price $5.30 domestic postpaid or $4.75 GPO Bookstore

Stock Number 0303-01100
ABSTRACT

The National Workshop on Building Practices for Disaster Mitigation was concerned with earthquakes, extreme winds, and similar dynamic hazards. These

proceedings present recommendations derived at the workshop and addressed to

policy makers in government and industry as well as practitioners in engineering, architecture, land use planning, and the earth and meteorological sciences.

The recommendations evaluate current building practices, define opportunities

for improving current practice from documented research findings, and recommend

research to fill gaps in knowledge. Recommendations are made for implementation

of improved practices at professional and policy levels. The objectives include

avoidance of human suffering, reduction of property loss, and maintenance of

vital function in buildings under conditions threatening disaster. Fifteen

review articles were prepared by knowledgeable individuals in the professions

and research disciplines to define the state of the art in disaster mitigation

and to guide discussions at the workshop; the articles are included in the

proceedi ngs

.

Key Words: Building; earthquakes; hazards; land use; natural disasters;

structural engineering; wind effects.

HI

CONTENTS

Page

Abstract i i

Introducti on 1

Natural Disasters 1 National Workshop on Building Practices for Disaster Mitigation ... 1 Cooperative Federal Program on Building Practices for Disaster

Mitigation 5 Future Actions 6 Workshop Recommendations 7

A. Policies for Disaster Mitigation 7 Actions at Governmental Levels 7

Al LandUsePlanning 7 A2 State Authority for Regulations 8

A3 Explicit Degree of Risk 8

A4 Safety of Existing Buildings 9 A5 Building Regulatory Personnel 9 A6 Review Boards 9 A7 Building Operators 9 AS Federal Disaster Assistance as Stimulus to Mitigation. . . 10

A9 Higher Standards in Post-Disaster Restoration 10

AlO Tax Relief 10

All Insurance Requirements 10

A12 Effective Mitigation in Federally Assisted Projects. ... 11

Al 3 State and Local Tax Incentives 11 Al 4 Hazard Mitigation in New Federal Building 12

A15 Correct Deficiencies in Government Buildings 12

A16 Eliminate Unsafe Buildings in Urban Renewal 12

Al 7 Local Disaster Emergency Plans 12

A18 Federal and State Assistance for Disaster Preparedness • . 13

A19 Survey Existing Buildings 13

A20 Post Disaster Surveys 13

A21 Benefit Cost Studies 14

A22 Technology for Disaster Mitigation 14

A23 Special Professional Skills 15

Actions by the Insurance Industry 15

A24 Disaster Insurance Program 15

Actions by the Design and Construction Community 16

A25 Professional Design Team 16

B. Practices for Disaster Mitigation 17

Actions by Professional Practitioners 17

Bl Seismi city Studies 17

B2 Minimum Design Loads for Earthquakes 17

B3 Design Wind Loads 18

B4 Dynamic Analysis 18

B5 Soi 1 -Structure Interaction 18

B6 Strength, Ductility and Damping 19

B7 Minimum Details 19

B8 Hazards from Adjacent Sites 19

V
B9 Hazards During Construction 20

BIO Design of Furnishings and Equipment for Disaster

Conditions 20

Bll Tie Downs for Mobile Homes 20

Actions by Standards Writers 21

B12 UpdatingSeismic Codes 21 B13 Recurrence Interval 21 B14 Site Effects 22

B15 Loading Duration 22

B16 Element Characteristics 22

B17 Debris Loads 22

Bl 8 Fi re Hazards 23

B19 Analytical Modelling 23

B20 Model Studies 23

B21 Explicit Levels of Performance 24

B22 Load Factors 25

B23 Reliability of Dynamic Analyses 25

B24 Drift Limitations 25

B25 Manuals of Practice 26

B26 Continuing Education 26

B27 Methodologies for Building Surveys 26

B28 Manual for Strengthening and Repair 27

B29 Data Gathering Procedures 27

B30 Forces on Equipment 28

B31 Manuals for Equipment 28

B32 Criteria for Utilities and Industries 28

Actions by Researchers 28

B33 Earthquake Risk Maps 29

B34 Site Effects in Earthquakes 29

B35 Instrumented Buildings 29

B36 Near Surface Meteorology 30

B37 Climatological Atlas 30

B38 Ultimate Dynamic Response 30

B39 Simplified Analyses for Design 30

B40 Laboratory and Field Experiments 31 B41 Field and Wind Tunnel Tests 31 B42 Rock and Soil Materials , 31 B43 Mechanisms of Failure 32

B44 Damping 32

f B45 Strength and Ductility Requirements 32

B46 Non-Structural Elements 33

Acknowledgments 34

Appendices 35

Appendix A - Workshop Program 35

Appendix B - Review Articles 40

Values and Costs

H. Kunreuther 41

Approaches to Implementation

P. Baseler 63

Earthquake Hazards for Buildings

H . Donovan 82

VI
The Problem of Seismic Zoning

S. Algermissen 112

Wind Hazards for Buildings

J. Vellozzi and J. Healey 126

Land Use Planning and Natural Disaster Mitigation

J. Wiggins, W. Petak, M. McCoy, D. Moran,

J. Slosson, and W. Monash 139

Architectural Approaches to Hazard Mitigation

E. Hillman and A. Mann 179

Procedures and Criteria for Earthquake Resistant

Des i gn (Part I

)

C. Pinkham 188

Procedures and Criteria for Earthquake Resistant

Design (Part II)

N. Newmark and W. Hal 1 209

Procedures and Criteria for Wind Resistant Design

J. Vellozzi and J. Healey 237

Criteria for Building Services and Furnishings

J. Ayers and T. Sun 253

Behavior of Structural Elements

B, Bresler 286

Behavior of Structural Systems Under Dynamic Loads

R. Sharpe, G. Kost and J. Lord 352

Survey and Evaluation of Existing Buildings

F. McClure 395

Abnormal Loading on Buildings and Progressive

Collapse

N. Somes 427

Appendix C - List of Participants 471

BUILDING PRACTICES FOR DISASTER MITIGATION

INTRODUCTION

Natural Disasters

Property losses from natural disasters have averaged approximately one billion dollars per year in the United States. This is the most easily

measured but perhaps the least important part of the losses. Losses from

human suffering, disruption of productive activities and expenditures in

disaster relief are difficult to quantify but magnify disaster losses many

fold.

The Disaster Preparedness Study , published in January 1972 by the Office

of Emergency Preparedness in response to Public Law 91-606, states: "Land-use

and construction regulations containing strong disaster mitigation features

can in the long run alleviate losses caused by natural disasters." Disaster

mitigation denotes preventive measures to reduce the damages caused by extreme

environments such as earthquakes and high winds and therefore to avoid the

chain reaction of failures which is called disaster. Buildings support and

shelter most human activities. Thus, preventing building collapse mitigates

most human suffering, preventing building damage greatly reduces property

losses, and continuing the functionality of buildings supports emergency

activities and prompt return of normal economic and social functions. These

benefits require consideration of disaster hazards in the practices of land

use planning, building design, construction, operation and maintenance.

Any building site is subjected to several types of natural hazards.

These may include earthquake and extreme winds as well as floods, landslides,

storm surges and conflagrations. All substantial risks should be considered

in design and use of a building. The National Workshop on Building Practices

for Disaster Mitigation held August 28 to September 1, 1972, focused on prompt

and effective approaches to disaster mitigation.

Earthquakes and extreme winds, as well as explosions and some accidents,

produce closely related dynamic loads and mobilize similar resistance mechanisms. Integrated attention to these hazards costs little more than treatment

of earthquakes alone and provides substantially increased benefits. The

building practices discussed at the workshop focus on these hazards. Although

not dealt with explicitly, flooding, landsliding, and storm surge hazards also

can be effectively mitigated by the land use planning and control practices

discussed.

The measures for disaster mitigation treated at the workshop can be

subdivided into Policies and Practi ces . Policies comprise executive or legislative actions of public authorities as well as the regulations of private organizations such as lending and insurance companies. Practices denote

the implementation of policies through the activities of design professions,

builders, and regulatory authorities. This report addresses the building

practices necessary for effective mitigation of the selected group of disaster

hazards and the policies needed to authorize their implementation.

National Workshop on Building Practices for Disaster Mitigation

This report, which constitutes the proceedings of the National Workshop,

reviews current building practices in the light of disaster mitigation and

recommends action for four distinct audiences.

1. Policy makers in Federal, State and local governments, to provide

guidance for laws, regulations, policies,^ and programs for

1
mitigation of hazards of natural disasters.

2. Practitioners, for guidance on the best current practices for

decisions which must be made at once.

3. Standards writers and those responsible for developing recommended

practices, for guidance on the deficiencies in present practices

which can be resolved by developments from wel 1 -documented research

f i ndi ngs .

4. Researchers, for guidance as to the unknowns which most severely

constrain improvements in building practices.

The recommendations were prepared at the workshop which brought together

the diversity of knowledgeable professional and regional interests required

for a balanced view of building practices for disaster mitigation. Most of

the participants, who are listed in Appendix C, are practitioners in planning,

design and supervision of construction because these are critical steps in

building for disaster resistance. However, some are researchers who increase

the level of knowledge about the loadings causing disasters and about the

response of buildings to these loadings. Most are engineers because the

topics are strongly involved with the physical response of buildings in

disasters and with design approaches to control of this response. Physical

scientists, architects, planners, social scientists, and public administrators

also provided their perspectives of disaster mitigation.

The discussions of the workshop were based upon review articles which

summarize the state of practice and research knowledge in the principal areas

of building practice. The review articles are contained in Appendix B.

The review article "Values and Costs" is by Howard Kunreuther, an economist, who has worked on the economics of natural disasters. The article

points out that acceptable levels of risk of disaster losses depend upon the

balance between the value of lesser risk and the cost of obtaining it. Potential losses include losses of physical property, indirect economic losses,

failure of buildings to perform functions of importance to the community, and

human suffering. The review describes approaches to a detailed evaluation of

these losses in terms which would improve the ability of decision-making

bodies to define what constitutes acceptable risk and to assess accordingly

present and future codes. The economic relations between disaster relief and

disaster mitigation policies also are developed.

The review article "Approaches to Implementation" is by Paul Baseler

and reflects his career experience in the building regulatory system. Better

building practices are effective only when improved procedures for land use planning, siting, design, construction, and operation are effectively implemented. The review evaluates approaches to the formulation, enactment, and

enforcement of land use and building regulations. Actions are considered for

many pertinent roles, such as legislative, professional planning and design,

construction management and labor, industrial, and governmental. Organizations

which may play important roles in improving building practices are identified,

their potential contributions described, and approaches to their participation

recommended. Specific attention is given to resolution of potential conflicts

and impediments to implementation of improved building practices.

The review articles by Neville Donovan, a foundation engineer, "Earthquake

Hazards for Buildings," and by S. T. Algermissen, a seismologist, "The Problem

of Seismic Zoning," describe our knowledge of the environment to which earthquakes may subject buildings. They consider effects of ground shaking and

permanent relative displacements arising from faulting, subsidence, and sliding. Practices accounting for expected amplitude and duration of shaking as functions of recurrence interval, geographical location, overburden characteristics, and topography are reviewed in light of research knowledge. The former

2
review also treats soi 1 -foundati on-structure interaction as a function of the

dynamic response cnaracteri sti cs of the structure.

The review article "Wind Hazards for Buildings," by Joseph Vellozzi and

John Healey, structural engineers, deals with the effects of geographical

location and local site conditions on extreme winds from hurricanes, thunderstorms, tornadoes, and extratropical storms. Attention is given to: wind

speeds as a function of recurrence interval, structure of wind turbulence,

variation of speed with height above ground, height of the planetary boundary

layer, lapse rates and thermal winds. The aerodynamic interactions of buildings and building elements with the wind are reviewed with attention to

overall and local external pressure coefficients for various body shapes, and

effects of openings on interior pressures. Current practices for design,

including use of wind tunnel tests, are related to documented research findings.

The review article "Abnormal Loading on Buildings and Progressive Collapse"

by Norman Somes, a research structural engineer, considers loads from explosions, collisions of vehicles and aircraft, unexpected failure of members and

other causes which may lead to general structural failure and disastrous loss

of function, property or life. Procedures and criteria accounting for these

loads in current building practices are evaluated in light of experiences with

these loads and documented research knowledge.

The review article "Procedures and Criteria for Land-Use Planning" is

prepared by a team of planners, public administrators, geologists and

engineers. It relates natural disaster hazards to site characteristics,

building uses, and community planning. Examples include soil conditions

which amplify earthquake motions at the natural frequencies of the building,

the presence of potentially active faults, susceptibility to subsidence or landsliding, and topography leading to severe wind exposure or exposure to

storm surge or moving flood water. Procedures and criteria for land use planning can forbid potentially hazardous uses or can require special efforts

to mitigate site hazards. The review of current practices characterizes the

approaches taken by current criteria and practices and evaluates their physical, economic, and social effectiveness.

The review article by Ernest Hi 11 man, Jr., structural engineer, and

Arthur Mann, architect, "Architectural Approaches to Hazard Mitigation" dis- cusses how building forms, spatial distributions, and interior designs can be

adapted to natural disaster hazards as part of the design to provide a functional environment in harmony with the natural environment. The review

evaluates organizations of the design team for synthesis of external and

internal forms which provide effective function and benign interactions with

earthquakes, extreme winds, explosions and similar causes of disasters.

Methods are noted for encouraging architectural approaches to disaster mitigation at the programming and schematic design stages.

Review articles "Procedures and Criteria for Earthquake Resistant Design"

are presented by C. W. Pinkham, structural engineer, and by N. M. Newmark and

W. J, Hall, research structural engineers. Experiences in recent earthquakes

have provided a laboratory for evaluation of present procedures and criteria

for earthquake resistant design. Design procedures and the criteria used with

them are reviewed for consistency and effectiveness in accounting for

loading magnitude, structural behavior, and the desired limitation of damage.

The reviews consider the degree to which procedures and criteria tend to

reinforce the designer's appreciation for the actual loading and response

characteristics and assist him in understanding limitations of the loading

and response models and effects of "non-structural" elements on structural

beha vi or

.

The review article "Procedures and Criteria for Wind Resistant Design"

by Joseph Vellozzi and John Healey, structural engineers, is based on recent

experiences with buildings subjected to severe winds as well as theoretical

and laboratory studies. Design procedures and the criteria used with them,

3
are reviewed as noted above for earthquake-resistant design. Design approaches are considered for hurricanes, tornadoes, thunderstorms, and extratropi cal

storms

.

The review article "Criteria for Building Services and Furnishings"

is prepared by J. Marx Ayres and Tseng Yao Sun, mechanical engineers.

Failures of equipment and services, such as elevators, luminaires, and piping,

and furnishings such as suspended ceilings and shelving often cause loss of

vital function, extensive property loss, and severe hazards to life. The

review defines mechanisms of failure induced by earthquakes, extreme winds,

explosions and similar causes, evaluates procedures and criteria for avoidance

of these failures, and discusses means for effective implementation. Specific

attention is given to both mechanisms which can be accounted for in criteria

for non-structural systems and those which require consideration paralleling

that of the primary structural system.

The review article "Behavior of Structural Elements" by Boris Bresler,

research structural engineer, evaluates design and construction procedures

and criteria intended to assure adequate strength and ductility in structural

members and connections. Consideration is given to expression of resistance

functions (relations of generalized force to corresponding generalized displacement) for repeated and reversed loadings as dependent on strain history,

rate of strain, and mechanism of failure. Simplified resistance expressions

for practical use in design are evaluated for range of validity and the

extent to which they illuminate the parameters and mechanisms most strongly

affecting the element behavior.

The review article "Behavior of Structural Systems under Dynamic Loads"

by Roland L. Sharpe, Garrison Kost, and James Lord, structural engineers,

evaluates methods for the analysis of the response of structure to the

dynamic loads which threaten damage in natural disasters. It presents first

a general review of analytical methods followed by a discussion of structural

configurations and modeling procedures. Applications of the analytical

methods are then presented, and many of the factors to be considered in the

dynamic analysis and design of a structure are discussed. As equipment and

systems supported in structures are often important to public safety, equipment-structure interaction and suggested analytical techniques are also

discussed. Finally, present philosophy in practice and trends in design are reviewed together with possible simplified design techniques.

The review article "Survey and Evaluation of Existing Buildings" is

prepared by Frank E. McClure, structural engineer. Buildings constructed

with earlier building practices require special evaluation of hazards from

earthquakes, extreme winds, and similar loadings in order to avoid future

disasters. In emergency and post emergency situations, evaluations of

deteriorated and damaged buildings are needed to determine whether they are safe for immediate use and whether repairs are economically feasible. Present procedures and criteria are reviewed and evaluated; recommendations are given for public policies and professional practices in survey and evaluation,

as well as needs for research on methods of strengthening and repair.

There is a strong emphasis on earthquakes and Californian experiences

in the review articles. Is this emphasis on earthquakes reasonable when property losses from wind damage to roofs, on the average, exceed earthquake

losses, and when lives lost in tornadoes exceed those in earthquakes? Likewise,

is the emphasis on Californian experiences valid in light of the fact that substantial risks of great earthquakes exist for areas east of the Rockies? It occurs because recent experience has shown severe inadequacies in the widely

accepted design practices for earthquake resistance and because the building

community in California has worked most effectively on these problems. However, the discussions and recommendations adopt a national view in identifying

best practices for earthquake and extreme winds, in recommending development of

improved practices, and in calling for effective focusing of research efforts.

4
Better building practices applied nationwide are needed for reduction of wind

losses and to end the potentially catastrophic neglect of earthquake hazards

in most of the country.

The recommendations were developed in two days of subcommittee meetings

following two days of intensive discussion of the problems as shown in the

workshop program in Appendix A. The following subcommittees took an interdisciplinary view of policies and practices for mitigation of specific hazards:

1. Implementation of Earthquake Hazard Reduction.

2. Formulation of Earthquake Hazard Reduction Practices.

3. Implementation of Wind Hazard Reduction Practices.

4. Formulation of Wind Hazard Reduction Practices.

On the fourth day, the subcommittees reviewed and commented upon each

others' preliminary reports. The second draft reports of the subcommitee,

which contained recommendations and comments, were discussed on the fifth day

of the workshop. The editors subsequently assembled the recommendations and

commentaries into the form presented here. This was followed by a final

review by all participants.

Cooperative Federal Program on Building Practices for Disaster Mitigation

Federal concern with building practices for disaster mitigation arises

from major statutory responsibilities. Approximately 37 percent of all new construction is directly or indirectly Federally supported; good practices

protect these investments and responsibilities. The Federal government is

obligated by PL 91-606 to pay for repair or replacement of all public facilities damaged in natural disasters; private nonprofit hospitals recently have

been added to this responsibility. Forgivable amounts of Federal loans to

private citizens for repair of disaster damages has approached $200 million

for the San Fernando earthquake of 1971.

However, Federal support for these building practices has not been

consistent with the potential benefits. Millions of Federal dollars have

been spent each year in research on basic mechanisms of earthquake and storms

and on the response of structures, but practicing engineers have generally

been left to synthesize this knowledge and develop improved building practices

with their own funds in their spare time. This gap between research and

practice has long been evident to both practitioners and researchers.

The Cooperative Federal Program in Building Practices for Disaster

Mitigation was initiated in the spring of 1972 by the National Science

Foundation and the National Bureau of Standards, following planning throughout the winter in conjunction with the Office of Emergency Preparedness and

the Department of Housing and Urban Development. The program seeks to

integrate extensive activities and resources in Federal agencies and work

with professional organizations, private practitioners, and state and local

governments in support of improved building practices.

The overall objectives of the program are: (1) to synthesize current

knowledge and develop improved building practices for insuring the safety of new and existing buildings with substantial human occupancy and (2) to make

this knowledge available in usable form to assist state and local officials

in effecting land-use planning and building regulation to mitigate the

effects of natural disasters.

This report is the first major output of the program.

5
Future Actions

The recommendations of the workshop call for substantial efforts in

development of improved practices and in important research. A greater

effort, if all costs are counted, is required to implement best current

practices and future improvements. Design engineers accustomed only to

concepts of static design for strength must become familiar with dynamic

structural responses and the parameters affecting the energy absorbing

abilities of buildings. Construction personnel and building inspectors must

understand how seemingly minor factors, such as connection details, vitally

affect structural resistance. Then, good practice will be meaningful rather

than a seemingly arbitrary nuisance. Administrators, planning groups, legislative bodies, and the using public must be made aware of the costs and

potential values of measures for disaster mitigation so they may make informed

decisions on policies affecting disaster hazards.

Activities in implementation of the workshop recommendations have begun.

The Defense Civil Preparedness Agency has joined the Cooperative Federal

Program to support development of methodology and criteria for evaluating

the safety of existing buildings. Discussions of further activities and

participation of additional agencies are proceeding.

6
WORKSHOP RECOMMENDATIONS

The recommendations contained herein represent the consensus of the

participants in the workshop, but do not constitute an individual

endorsement by any participant or his organization.

A. POLICIES FOR DISASTER MITIGATION

The number of groups involved in the building process is large and their

interests are varied. All these groups, however, must be involved in any

implementation program to effectively mitigate losses from future natural

disasters. Such an implementation program must consider changes that will

effect current practices in design and construction, changes that will give

new tools to policy makers and political jurisdictions so that risks can be

assessed in policy making, and changes that will effect the many regulatory

systems so that new policies can be carried out and the public protected.

The overall objective of this implementation program is the mitigation of

future losses of life and property.

Actions taken by governmental bodies at the Federal, State, and local

levels, as well as by the professional, financial and insurance communities

can play a significant role in achieving disaster mitigation through improved

land-use planning, building design, and construction practices. The Federal

government through its own direct construction programs and through its many

grants, loans, insurance assistance and regulatory programs can lead the way

towards implementing practices which recognize risks of natural disasters and

plan for mitigating losses. However, it must be recognized that State governments have the constitutional responsibility in this field of health and

safety and they in conjunction with local governments must take action to

implement disaster mitigation programs with the Federal government providing

support and assistance when needed. Governmental bodies at all three levels

require the active participation and expertise of a wide spectrum of disciplines in design and engineering, the physical and social sciences, and the

banking and insurance industries.

These recommendations were formulated to define the changes necessary to

implement a national program for mitigating losses from natural disasters.

While few are exclusive to a particular group, they have been divided for

convenience into those requiring prime action at governmental levels, by the

insurance industry, and by the professional design and construction community.

Actions at Governmental Levels

Land use plans and building regulatory measures based on a consideration of

the risks associated with natural disasters provide an effective means for

hazard mitigation . Comprehensive land-use plans (general plans, master

plans) should include hazard mitigation sections similar in content to the

"Seismic Safety Element" for earthquakes included in the California General

Plan Law enacted in 1971. (Section 65302 of the Government Code [California]

)

Such measures will assure that the citi zenry and their elected representatives

are informed of the risks associated with all aspects of natural disasters.

RECOMMENDATION Al : LAND USE PLANNING

A NATIONAL PROGRAM SHOULD BE ESTABLISHED TO

COORDINATE LAND USE PLANNING FOR DISASTER

7
MITIGATION. IT SHOULD BE DIRECTED TOWARD

STATE AND LOCAL GOVERNMENT IMPLEMENTATION.

An effective system of planning for disaster mitigation must consider land

use planning at the State, regional, and local levels. Planning should be

undertaken by qualified professionals. Plans must be presented in a manner

easily understood and used by all parties. A national program in this area could provide coordination of the planning processes and assure reasonable

consistency among the States. The establishment of a clearinghouse for the

dissemination of design criteria and standards for land use planning should be

included in the program.

RECOMMENDATION A2 : STATE AUTHORITY FOR

REGULATIONS

STATE GOVERNMENTS SHOULD ESTABLISH MINIMUM RULES

AND REGULATIONS TO PROVIDE FOR THE SAFETY OF THE

PUBLIC FROM THE HAZARDS OF NATURAL DISASTERS.

THEY SHOULD REQUIRE INCORPORATION OF THESE PROVISIONS IN LOCAL LAND USE PLANNING AND BUILDING

REGULATORY MEASURES.

State governments in exercising their constitutional authority for the promulgation of codes should develop provisions which recognize the hazards of

natural disasters. Since such disasters usually occur over a geographical

area encompassing several local governmental jurisdictions and the effects in one locality may influence neighboring communities; comprehensive area-wide

planning to mitigate these effects is necessary. Local land use plans and

building codes should incorporate provisions dealing with natural disasters

and other hazards. These provisions should be applicable not only to structures with substantial human occupancy but to all potentially hazardous

facilities. States should require coordination of codes and ordinances in

comprehensive plans to assure consistent area wide standards. The States

should also consider the protection of vital emergency services required

during and after the occurrence of natural disasters in setting minimum

standards with respect to land use planning, building codes and emergency

planning.

RECOMMENDATION A3: EXPLICIT DEGREE OF RISK

CODES, STANDARDS AND CONTRACTS FOR ARCHITECTURAL

AND ENGINEERING SERVICES SHOULD CONTAIN LANGUAGE

SETTING FORTH THE DEGREE OF RISK ACCEPTED IN THE

DESIGN PROCESS.

The building codes now in effect in earthquake-prone areas are not intended

to prevent damage to buildings during major earthquakes. Moreover, good

current practice as described in the workshop papers implies a risk of damage.

This risk is not well recognized by the public nor by most building owners,

and is not clearly stated in architectural and engineering contracts. Accordingly, while it is generally held that it usually is not feasible to design

buildings so as to have no damage during natural disasters when extreme loads

are experienced, the engineer and architect find themselves open to criticism

and even lawsuits if damage occurs. Because the situation is not openly

recognized, the architect or engineer is seldom able to discuss the risk, and

possible steps to reduce the risk, with his client. Often, risk of damage

can be significantly decreased by improved practices, sometimes at little

increase in construction cost, but the necessary engineering studies are not

considered nor included within the usual services. Studies to achieve a lesser risk should be recognized as an important additional service.

8
RECOMMENDATION A4: SAFETY OF EXISTING

BUILDINGS

LAND USE REGULATIONS AND BUILDING REGULATORY

MEASURES SHOULD INCLUDE PROVISIONS FOR EVALUATION

OF THE SAFETY OF EXISTING BUILDINGS AGAINST

NATURAL HAZARDS AND REHABILITATION OF UNSAFE

BUILDINGS.

In addition to insuring the adequacy of new construction, it is important to ascertain the safety of the large inventory of existing buildings. The City

of Long Beach, California, Ordinance C-4950, "Earthquake Hazard Regulations

for Rehabilitation of Existing Structures Within the City" serves as a useful

guideline in this respect. It also sets standards for rehabilitation of

existing unsafe buildings.

RECOMMENDATION A5 : BUILDING REGULATORY

PERSONNEL

STATE GOVERNMENTS SHOULD ESTABLISH CRITERIA

AND PROGRAMS FOR QUALIFICATION AND CERTIFICATION OF BUILDING REGULATORY PERSONNEL.

Improved inspection and enforcement of building regulations are essential to

implementation of practices for disaster mitigation. The skills needed for

various activities of the regulatory bodies should be identified. Professional

degree programs, two-year technology programs, trade school courses, continuing

education seminars, and in-service training should be employed to upgrade

current personnel. Salaries should be reviewed to permit retention and

attraction of properly qualified personnel.

RECOMMENDATION A6 : REVIEW BOARDS

LEGISLATIVE BODIES ADOPTING BUILDING CODES AND

LAND USE REGULATIONS SHOULD CREATE BY STATUTE

"REVIEW BOARDS" TO HEAR REQUESTS FOR VARIANCES

AND APPEALS FROM DECISIONS OF REGULATORY

OFFICIALS.

The "Review Board" would provide a means for obtaining consistent procedures

for the review of investigations, design criteria, and plans which are not

within the scope of existing regulations. This review procedure should assure that appropriate levels of disaster protection, consistent with the

risk, are attained where exceptions are granted. The legislation should

define the authority of the "Review Board" and the professional qualifications

of its members. Professional societies should advise in the appointment of

board members

.

RECOMMENDATION A7: BUILDING OPERATORS

STATE GOVERNMENTS SHOULD REQUIRE THAT BUILDING

OPERATORS BE TRAINED FOR DISASTER PREPAREDNESS

AND OPERATIONS.

Building operators should be trained to help mitigate disasters by preventive

maintenance procedures, keeping as-built record drawings updated, and following through on routine safety tests of mechanical, electrical, elevator, and

fire protection systems. They should be familiar with their responsibilities

in state and local disaster plans.

9

494-610 O - 73 - 2
Governmental programs can provide appropriate incentives for improved practices for disaster mitigation

.

RECOMMENDATION A8 : FEDERAL DISASTER ASSISTANCE

AS STIMULUS TO MITIGATION

THE FEDERAL DISASTER ASSISTANCE PROGRAM

SHOULD BE REFORMULATED TO PROVIDE INCENTIVES

FOR PREVENTIVE DISASTER MITIGATION EFFORTS AND

TO REDUCE POST-DISASTER INCENTIVES FOR IGNORING

THE RISKS ASSOCIATED WITH A SPECIFIC STRUCTURE

AND SITE.

The Federal Disaster Assistance Program, as presently constituted, provides

liberal post-disaster benefits in such form as temporary housing, low cost

loans and employment compensation to individuals, and rehabilitation of public

facilities. Individuals and communities have come to rely on governmental

help after a disaster, rather than to develop an awareness of disasters and

take feasible preventive measures. In essence, the program is weakening

preparedness and mitigation efforts. While some governmental aid is clearly

indicated on compassionate grounds, the assistance program needs to be re- evaluated so that it provides a stimulus and not an impediment to predisaster

mitigation efforts by individuals and communities.

RECOMMENDATION A9 : HIGHER STANDARDS IN POSTDISASTER RESTORATION

THE FEDERAL DISASTER ASSISTANCE PROGRAM SHOULD

REQUIRE THAT POST- D I SAST ER REHABILITATION AND

REPLACEMENTS TO DAMAGED FACILITIES BE DESIGNED

AND CONSTRUCTED WITH A VIEW TOWARDS MITIGATING

FUTURE LOSSES.

The Federal Disaster Assistance Program currently in effect calls for the

rehabilitation and replacement of buildings damaged or destroyed by a declared disaster in accordance with existing land use and codes. Speed in

providing aid is stressed. Consequently, structures are rebuilt on the same

sites under codes that may not reflect the latest technological advances.

Such structures are therefore exposed to the same pre-disaster risks and may

have to be rebuilt at public expense in the case of a new disaster. This

condition should be corrected in re-evaluating the Federal Disaster Assistance

Program

.

RECOMMENDATION AlO: TAX RELIEF

TAX REGULATIONS SHOULD BE REVIEWED TO

IDENTIFY FEATURES THAT WOULD PROVIDE

EQUITABLE TAX RELIEF TO ENCOURAGE INSURANCE

FOR LOSSES IN NATURAL DISASTERS.

For infrequent hazards such as earthquakes, the tax regulations could provide

for a longer tax averaging period in order to build larger insurance reserves

for these infrequent natural hazards.

RECOMMENDATION All: INSURANCE REQUIREMENTS

THE BANKING REGULATORY AGENCIES SHOULD REQUEST

10
THAT THE AMOUNT OF MONEY LOANED OR GUARANTEED

BE INSURED AGAINST NATURAL DISASTERS.

Such actions at the Federal and State level regarding insurance would provide

far-reaching incentives for disaster mitigation. They could be accomplished

within existing, already legislated programs to stimulate owners and local

communities to upgrade building practices and lower insurance premiums.

RECOMMENDATION A12: EFFECTIVE MITIGATION IN FEDERALLY ASSISTED PROJECTS

THE FEDERAL GOVERNMENT SHOULD EMPLOY ITS

FINANCIAL ASSISTANCE AND LOAN INSURANCE PROGRAMS TO ENCOURAGE ENACTMENT OF EFFECTIVE LAND

USE PLANNING REGULATIONS AND BUILDING CODES,

AND THEIR STRICT ENFORCEMENT. APPLICATIONS

FOR FEDERAL FUNDS SHOULD BE REVIEWED TO INSURE

THAT CONSIDERATION HAS BEEN GIVEN TO RISKS FROM

NATURAL DISASTERS.

Many local programs are dependent on financial assistance from the Federal

government. About forty percent of the building activities of the country " are influenced by the Federal government. A condition for granting such

assistance should be the adoption of hazard mitigation measures, as recently

prescribed by HUD in regard to flood insurance protection in those communities

which have joined the National Flood Insurance Program. This policy could be

extended to withholding of Federal assistance from State and local communities

(after notification to comply) if disaster mitigation elements are not enacted

as part of the comprehensive planning process.

The Federal guidelines for evaluation, review, and coordination of Federal

and Federally assisted programs and projects, as issued by the Office of

Management and Budget, Executive Office of the President (Circular #A-95),

require that applications for Federal assistance be coordinated by an officially designated State or Metropol i tan/ Regi ona 1 Clearinghouse. The

Clearinghouse is charged with the responsibility for review and comment on the application to ascertain whether or not the project is consistent with

area wide plans and programs, and that overlap and duplication in the planning

process has been minimized. The Clearinghouse should expand its review to

insure that the siting and design of the project have considered the risks

from natural disasters.

RECOMMENDATION Al 3 : STATE AND LOCAL TAX

INCENTIVES

EFFECTIVE TAX AND OTHER INCENTIVES AT THE STATE

AND LOCAL GOVERNMENTAL LEVELS SHOULD BE DEVELOPED

TO ENCOURAGE DISASTER MITIGATION IN PLANNING AND

CONSTRUCTION.

Such incentives or penalties may be included in the tax systems, rather than

allowing the continuance of the present system where the improvement of

property with a view toward mitigating losses from natural disasters usually

increases the tax burden.

Governmental organizations can provide a strong stimulus to disaster mitigation by setting an example through their own actions . While some agencies

have initiated such policies, more positive moves in this direction are feasible and should be undertaken.

1 1
RECOMMENDATION A14: HAZARD MITIGATION IN NEW

FEDERAL BUILDING

THE FEDERAL GOVERNMENT SHOULD INCORPORATE HAZARD

MITIGATION CONSIDERATIONS IN ITS BUILDING AND

LEASING OPERATIONS.

It is feasible and most cost effective to initiate preventive measures prior

to occupying new facilities. Buildings for Federal use should be individually

investigated to assess their anticipated performance in natural disasters.

Evaluation reports must be prepared on a consistent basis using nationally

established guidelines.

RECOMMENDATION Al 5 : CORRECT DEFICIENCIES IN GOVERNMENT BUILDINGS

FEDERAL, STATE, AND LOCAL GOVERNMENTS SHOULD

IMMEDIATELY COMMENCE PROGRAMS TO CORRECT

DEFICIENCIES IN BUILDINGS THEY OCCUPY.

Many existing structures pose potential hazards in the event of a natural

disaster. Remedial action by government agencies for structures they are responsible for or occupying would considerably mitigate future losses.

Vital facilities necessary for providing immediate post-disaster relief

such as hospitals, emergency communication centers, schools and other

shelters, etc. should be given high priority.

RECOMMENDATION Al 6 : ELIMINATE UNSAFE BUILDINGS

IN URBAN RENEWAL

THE ELIMINATION OF OLD AND POORLY MAINTAINED

BUILDINGS POSING SUBSTANTIAL HAZARDS IN THE

EVENT OF A NATURAL DISASTER SHOULD BE ONE OF

THE IMPORTANT FACTORS CONSIDERED IN URBAN

RENEWAL PROGRAMS.

Urban renewal programs provide an excellent opportunity to abate clearly

hazardous buildings in the current inventory.

RECOMMENDATION Al 7 : LOCAL DISASTER EMERGENCY

PLANS

LOCAL GOVERNMENTS SHOULD DEVELOP COMPREHENSIVE

DISASTER PREPAREDNESS PLANS WHICH INCLUDE MEANS

FOR PROVIDING LIFE SUPPORT ACTIVITIES DURING

AND FOLLOWING A NATURAL DISASTER, REPAIR AND

REHABILITATION OF DAMAGED FACILITIES.

Local governments have a responsibility to provide emergency services including supplies, electric power, sites for debris disposal, etc. following a disaster. Contingency plans considering sources of supply and logistical

problems must be made prior to occurrence of a disaster. Means must be

established to insure that post disaster repair and remedial work is carried

out in accordance with accepted standards and certified by local building

authorities. The review article by McClure describes some of the provisions

already in existence for rehabilitation following an earthquake.

12
RECOMMENDATION A18: FEDERAL AND STATE ASSISTANCE

FOR DISASTER PREPAREDNESS

FEDERAL AND/OR STATE ASSISTANCE SHOULD BE

PROVIDED TO LOCAL GOVERNMENTS FOR DEVELOPING

AND IMPLEMENTING DISASTER PREPAREDNESS PLANS

AND CONDUCTING POST DISASTER DAMAGE ASSESSMENTS.

Both financial and nonfinancial assistance should be provided. The identification and acquisition of sites and equipment for use in the event of a disaster, funds to permit building inspection and consulting engineering

services for damage assessment, and coordination of disaster response activities between local jurisdictions fall in these categories. A re-evaluation

of funding allocated to civil preparedness programs should be made to assure

most effective use.

An effective program of disaster mitigation requires a data bank of documented

information and appropriate standards and criteria for building practices

.

RECOMMENDATION Al 9 : SURVEY EXISTING BUILDINGS

A SURVEY SHOULD BE MADE OF THE INVENTORY OF

EXISTING BUILDINGS TO ESTABLISH THE POTENTIAL

LOSSES IN NATURAL DISASTERS. FEASIBLE ALTERNATIVES, INCLUDING REHABILITATION OR REMOVAL,

SHOULD BE DEVELOPED FOR REDUCING RISKS FROM

HAZARDOUS STRUCTURES IDENTIFIED IN THE SURVEY.

Hazard mitigation for new construction can be achieved through use of up to

date standards and specifications. However, many existing structures were built prior to incorporation of hazard mitigation provisions in codes or in

accordance with codes which have since been updated. It is essential to

identify seriously hazardous structures and to take appropriate remedial

action. The survey results should be presented in a form clearly indicating

the probabilities of losses. Specific attention should be given to life

safety hazards posed by nonstructural elements. Results of the survey,

analyses, and evaluations, and recommended remedial actions must be effectively put before the appropriate officials, in both general government and

building control fields, public utilities corporations, the general public

and the media. The presentation to each should be tailored to their responsibilities and interests.

RECOMMENDATION A20: POST DISASTER SURVEYS

A COORDINATED PROGRAM INVOLVING FEDERAL,

STATE, AND PROFESSIONAL ORGANIZATIONS SHOULD

BE ESTABLISHED FOR CONDUCTING POST DISASTER

SURVEYS

.

The occurrence of a natural disaster presents an excellent opportunity to gain

important engineering, scientific and social data. The engineering and

scientific data are useful for developing improved disaster mitigation practices and policies. The social data will help define losses of productivity,

and tax base, and psychological and economic impacts on individuals and

communities. There has been insufficient coordination, in the past, between

post event investigators. Contingency plans and procedures should be established to collect in a comprehensive, non-dupl i cati ve manner the information

made available by the occurrence of a natural disaster. Such plans should

include provisions for the development of base line information prior to the occurrence of a disaster.

13
RECOMMENDATION A21 : BENEFIT COST STUDIES

THE FEDERAL GOVERNMENT SHOULD INITIATE STUDIES

AND PROGRAMS TO DEVELOP AND COLLECT DATA CONCERNING THE MANY LESS IMMEDIATE AND OFTEN

INTANGIBLE COSTS OF DISASTERS: LOSS OF PRODUCTIVITY, LOSS OF TAX BASE, PSYCHOLOGICAL AND

ECONOMIC IMPACTS ON INDIVIDUALS AND COMMUNITY,

ETC . Benefit-cost studies can provide valuable data inputs for developing and

implementing a hazard reduction program. It may not be economically feasible

to mitigate completely the effects of natural hazards. Hence it may be

necessary to decide how much of our resources should be devoted to mitigating

natural disasters as compared to other hazards, and for choosing the most

effective disaster mitigation methods.

Benefit-cost studies themselves do not make decisions. They are a tool for

digesting many diverse facts and for portraying implications of alternative

strategies. The usefulness of such studies is related directly to the

validity and completeness of the input. The efforts noted must be launched

immediately to provide the input necessary for studies that should be made in

the near future.

In initiating the recommended studies and programs, it is desirable that

feasibility studies be made prior to commencing the major activity. The

feasibility studies should ascertain the type of data to be collected and

generated, and the use of the data. It is essential, however, that initial

efforts using simple (and possibly crude) measures of the hazards to buildings

be completed as soon as possible. These would be continually required as more detailed compilations and methodologies are undertaken and completed.

Kunreuther in his review article presents an extended discussion of the

usefulness of benefit-cost studies, of perspectives that must be kept in

mind, and of steps required to implement such studies.

RECOMMENDATION A22: TECHNOLOGY FOR DISASTER

MITIGATION

THE FEDERAL GOVERNMENT SHOULD ESTABLISH A NATIONAL FOCAL POINT AND INFORMATION BANK TO

PROVIDE STATE AND LOCAL PLANNING AND REGULATORY

BODIES AND THE DESIGN PROFESSIONS WITH NEEDED

INFORMATION RELATED TO DISASTER MITIGATION.

The national program would use mul ti -professional expertise to develop recom- mendations dealing with:

a. Standard approaches to establishing levels of risk for design.

b. Design concepts for disaster mitigation.

c. Coordination of disaster research and investigations.

d. The use of a referral system to data and information.

The bank should contain the latest information on disaster mitigation. Critical information would then be readily available to those engaged in planning

for disasters. The bank should contain:

a. A referral system to data centers containing techni cal -phys i cal data

necessary for planning (e.g., geologic, hydrologic, seismic, meteorologic, etc.).

14
b. Information pertaining to the latest and most comprehensive

ordinances and regulations.

c. Information necessary for land use planning.

Continued effort must be put forth in developing better decision making tools

which inform the legislator and the citizen of risk alternatives so that

decisions pertaining to disaster mitigation can be made by the appropriate

level of government. The tools will help achieve a balance between investments providing safety from disaster and those providing other social benefits

for the community.

RECOMMENDATION A23: SPECIAL PROFESSIONAL SKILLS

A NATIONAL FOCAL POINT SHOULD BE ESTABLISHED

TO ASSIST THE STATE PROFESSIONAL LICENSING

AUTHORITIES IN PREPARING AND MAINTAINING

GUIDELINES DEFINING SPECIAL SKILLS FOR THE

PROFESSIONS INVOLVED IN DISASTER MITIGATION

DESIGN AND CONSTRUCTION PRACTICES. SUCH A FOCAL POINT SHOULD UTILIZE CONTINUING INPUT

FROM TECHNICAL REVIEW BOARDS COMPOSED OF

REPRESENTATIVES FROM THE RELATED PROFESSIONAL

SOCIETIES

.

It is interesting to note that, in some professional areas (e.g. medicine),

there are standards for peer groups to evaluate the competence or ability of

professionals to perform specialized services. It is essential that guides,

prepared by a nationally recognized objective group with continuing input

from peer professional groups, be available to the States for use in formulating standards for professional practices in areas where there is a potential

for exposure to natural hazards.

Actions by the Insurance Industry

The increasing demands for nega ti ve incentives for disaster mitigation provided by Federal and local governments (through low interest loans and

forgiveness clauses ) can be curtailed through an effective nationwide disaster

insurance program. Such a program can also stimulate improved building design

and construction to mitigate future losses .

RECOMMENDATION A24: DISASTER INSURANCE PROGRAM

INSURANCE COMPANIES SHOULD ESTABLISH A NATIONWIDE DISASTER INSURANCE PROGRAM WHICH IS

CONDUCIVE TO DISASTER MITIGATION. A POTENTIALLY

VALUABLE FEATURE WOULD BE A DIFFERENTIAL RATE

STRUCTURE WHICH RECOGNIZES DISASTER RISKS AND

ENCOURAGES IMPROVEMENTS IN LAND USE PLANNING,

DESIGN, CONSTRUCTION, MAINTENANCE, AND

RESTORATION

.

Risks of earthquakes, extreme winds and similar dynamic hazards are broadly

distributed and annual losses are generally less than the fire loss. An

insurance program is practicable and would assign costs to the potential

beneficiaries. The appropriate Federal role as reinsurer or administrator

merits consideration. Thus, it provides an incentive for improved practices

for disaster mitigation in contrast to the negative incentives of disaster

relief measures. Differential rate setting for fire insurance has been

15
effective in mitigating fire losses by providing incentive for improvement

of fire defenses, and should be similarly effective for natural disasters.

Insurance payments for restoration or repair following a disaster should

permit modifications which upgrade the building, thus mitigating future

losses. Rates can reflect risks of extreme loads, and the qualities of:

land use regulations, building codes, local enforcement practices, mitigating

design features, and implementation in construction. The methodologies for

probable loss studies, essential for rate setting, are feasible and may be

developed with inputs from the inventories of existing buildings and benefit

cost studies cited above.

Actions by the Design and Construction Community

Improvements in the design process , including the factors to be considered by

the professional in planning and executing the design and the subsequent

construction of the facilities in accordance with the design, can play a significant role in mitigating losses from natural disasters . It is essential

that the natural hazards be recognized in the planning , design and construction

processes .

RECOMMENDATION A25: PROFESSIONAL DESIGN TEAM

AGREEMENTS FOR PROFESSIONAL DESIGN SERVICES SHOULD

CALL FOR AN INTERDISCIPLINARY DESIGN TEAM WITH THE

EXPERTISE REQUIRED TO DEVELOP DESIGNS TO THE

PERFORMANCE LEVELS NEEDED FOR DISASTER MITIGATION.

THE SERVICES OF THE DESIGN TEAM SHOULD INCLUDE

SURVEILLANCE OF CONSTRUCTION TO ASSURE IMPLEMENTATION OF THE PLANS AND SPECIFICATIONS AS WELL AS

RECOGNITION OF UNFORESEEN CIRCUMSTANCES DISCLOSED

DURING CONSTRUCTION. THE DESIGN TEAM SHOULD

CERTIFY COMPLIANCE OF THE DESIGN AND CONSTRUCTION

DOCUMENTS WITH APPLICABLE CODES AND REGULATIONS

AND THAT ALL PROFESSIONAL WORK WAS AT LEAST IN ACCORD WITH STANDARD PRACTICES.

Design for mitigation of disaster hazards provides buildings which will perform properly in an extreme environment. The acceptance of finite risks of

loss of function, property damage, and human suffering requires professional

inputs from a number of disciplines to fit the various aspects of building

design to its site and occupancy conditions. For instance, a major building

or development in a seismic area requires an engineering geologist, foundation

engineer, engineering seismologist, civil engineer, and a planner or architect

for site evaluation and hazard identification. The design and surveillance of

construction require an architect, structural engineer, mechanical engineer,

electrical engineer, foundation engineer, inspector, and testing laboratory

engineer. Each of these should have recognized authority in his area of the

design and surveillance as indicated by his certification of the design and

construction documents. The needs for these services and their complexity

should be recognized in the agreement for remuneration for professional

services. The effectiveness of surveillance of construction by the design

team is well documented by successes of the Field Act in California and

Chapter 70 of the Uniform Building Code. However, this surveillance should

not lessen the contractor's responsibility for quality control and compliance

with plans and specifications. The duties and their limitations of all members

of the design and construction teams should be clearly understood by all parties. Certification of compliance by the contractor that all work was in

accordance with the construction documents, and applicable codes and regulations should be required. o?

16
B. PRACTICES FOR DISASTER MITIGATION

Immediate and long range efforts by professional practitioners,

standards writers, and researchers are required to provide an integrated

development of building practices to mitigate the effects of natural disasters.

The recommendations in this section are directed to these three groups. Professional practitioners, including land-use planners, architects, engineers

and geologists, can implement immediately best current practices in present

activities for design and analysis of new structures and the evaluation and

strengthening of existing structures. Standards writers can effect improvements in practices by updating standards to the level of current knowledge and

including new information as it becomes available from accepted research findings. Standards generating organizations, both private and public, will be

major participants in this development work. Researchers can substantially

improve hazard mitigation capabilities by providing new information to fill

gaps in existing knowledge.

Actions by Professional Practitioners

Design loads used in current practice for extreme environments should reflect

all usable information on factors affecting the response of the buildings.

RECOMMENDATION Bl : SEISMICITY STUDIES

SEISMICITY STUDIES SHOULD BE CONDUCTED FOR

IMPORTANT STRUCTURES LOCATED IN SEISMIC ZONES

TO DETERMINE THE GROUND MOTIONS TO BE EXPECTED

AND THE INFLUENCE OF LOCAL SOIL CONDITIONS ON

THESE MOTIONS.

As pointed out in the review article by Donovan, both the general geographic

region and the local site conditions should be considered in evaluating the

effects of earthquakes on structures. Regional data on geologic conditions

and the available seismic record may be used to estimate expected magnitudes

and recurrence intervals. Local soil conditions may significantly influence

wave propagation and surface motions.

RECOMMENDATION B2 : MINIMUM DESIGN LOADS

FOR EARTHQUAKES

MINIMUM DESIGN LOADS FOR EARTHQUAKES SHOULD BE

BASED ON THE SEISMIC RISK MAP CONTAINED IN THE

1970 UNIFORM BUILDING CODE BUT WITH AREAS PRESENTLY DESIGNATED AS ZONE 0 CHANGED TO ZONE 1.

This recommendation should be considered as an interim measure for disaster

mitigation since knowledge obtained from recent earthquakes can be used to

update existing codes. Since historical earthquake data is limited and

existing risk maps are not based on recurrence interval, it is reasonable,

considering the uncertainties involved, to provide some protection for all

structures and upgrade Zone 0 to Zone 1. In general, designs with adequate

wind resistance will meet Zone 1 seismic requirements providing that major

structural components are adequately interconnected, i.e., roof to walls and

walls to foundation.

1 7
RECOMMENDATION B3: DESIGN WIND LOADS

DESIGN LOADS FOR WIND SHOULD BE B/^SED ON THE

1972 EDITION OF THE AMERICAN NATIONAL STANDARD

A-58.1, "BUILDING CODE REQUIREMENTS FOR MINIMUM

DESIGN LOADS IN BUILDINGS AND OTHER STRUCTURES."

This new standard incorporates a number of aspects of wind loading not

included in other standards. For example, the use of alternate recurrence

intervals (50 yr wind or 100 yr wind) in determining the wind loading permits

selection of the risk accepted in the design. In addition, design loads are based on the velocity and duration of the wind (gust factors) as well as characteristics of the structure (shape factor) and site (site roughness

effects). Maps indicating extreme wind characteristics permit consideration

of forces produced by hurricanes and extra-tropical storms. Since these

loadings account for recurrence interval, the traditional 33 percent increase

in allowable stress should be used only where it is justified by the loadduration sensitivity of material resistance as it is with strength of timber

elements. Although detailed wind speed data are not available and code

provisions do not cover tornadoes, their effects should be considered for

important structures in tornado prone areas. Until sufficient data are obtained to develop standards, considerable protection can be achieved using

appropriate detailing procedures to insure that all elements of the structure

are adequately interconnected. Details for seismic loading from the Uniform

Building Code or wind detailing requirements from the South Florida Building

Code may be used as guidelines.

It is important that procedures used in the analysis of structures subjected

to natural hazards reflect the dynamic character istics of the loading and

system response.

RECOMMENDATION B4 : DYNAMIC ANALYSIS

EARTHQUAKE AND EXTREME WIND RESPONSE ANALYSES

FOR STRUCTURES OF UNUSUAL SHAPE OR STRUCTURAL

CHARACTERISTICS SHOULD INCLUDE A DYNAMIC

ANALYSIS

.

Current practice as well as code provisions for both earthquakes and extreme

winds are based on a quasi-static approach. In some cases, standards and

codes identify structures requiring a more refined analysis. The ANSI

Standard, for example, identifies wind sensitive structures for which gust

action and dynamic "response are important as buildings taller than 200 feet

and having a height to least width ratio greater than 4. Until firm guidelines are established, designers should select structures in this category

on the basis of familiarity with the assumptions and limitations of the

quasi-static analyses used in current practice. Code provisions which would

identify structures requiring dynamic analysis for earthquake design are discussed in the review article by Pinkham and specified in a recent change

to the City of Los Angeles Building Code.

RECOMMENDATION B5 : SOIL-STRUCTURE INTERACTION

INTERACTION OF THE S 0 I L - STRUCTUR E SYSTEM SHOULD

BE INCLUDED IN THE SEISMIC ANALYSIS OF IMPORTANT

STRUCTURES.

Consideration should be given to the soil effects on the natural frequency

and the damping of the soi 1 -f oundati on-structure system as discussed in the

review articles by Sharpe, Kost, and Lord, by Newmark and Hall, and in the

ci ted references

.

18
Cons ideration should be given in the planning stages and in design to the

real nature of the potential hazards and their total effect on buildings.

RECOMMENDATION B6: STRENGTH, DUCTILITY, AND

DAMPING

DESIGNS FOR EARTHQUAKES, EXTREME WINDS AND

EXTREME LOCAL LOADS SHOULD CONSIDER THE DYNAMIC

RESPONSE PARAMETERS OF THE STRUCTURE AND THE

NEED FOR PROVIDING ENERGY ABSORPTION CAPABILITIES

AS WELL AS ADEQUATE STRENGTH. COMMENTARIES FOR

CURRENT STANDARDS AND SPECIFICATIONS SHOULD BE

USED TO INSURE PROPER INTERPRETATION AND USE OF

DESIGN CRITERIA IN THIS REGARD.

Although current practice involves the use of equivalent static loads, the

response of structures to earthquakes and extreme winds is related to the

dynamic characteristics of the individual components and the overall system.

Material properties under dynamic loads, repeated excursions into the inelastic range and large amplitude displacements are involved. Due consideration

should be given to those factors not usually considered in designs for dead

and live load. Particular attention should be given to the structural con- figuration to insure provision of adequate stiffness, ductility, and continuity. Failures encountered in previous natural disasters and progressive

collapses indicate the importance of connection details. Designs for connections should insure adequate resistance to the induced forces and transferal

to all load carrying members throughout the structure.

RECOMMENDATION B7: MINIMUM DETAILS

DESIGN CRITERIA FOR STRUCTURES FOR WHICH

PROFESSIONAL DESIGN ATTENTION IS NOT REQUIRED

SHOULD INCLUDE STANDARD MINIMUM DETAILS

SELECTED ON THE BASIS OF THEIR PROVEN ABILITY

TO WITHSTAND EXTREME LOADS.

The type of damage occurring in non-engineered structures is discussed by

Sharpe, Kost and Lord. Surveys following natural disasters, for example

inspections of damage done by Hurricane Celia in 1970, indicate that a major

portion of the damage could have been prevented had proven structural details

been used in the affected buildings. When a detailed analysis is not economical, minimization of life loss and property damage must be achieved by

eliminating potentially hazardous conditions, providing anchorage of various

components and adequate connection details. Anchorage requirements in the

Uniform Building Code and the South Florida Building Codes should be employed. These requirements should be reviewed using the new knowledge obtained

from analysis of the damage to dwellings caused by the 1971 San Fernando

earthquake. Recommended framing and details for wind damage mitigation in

wood frame houses are also available in Research Paper FPL 33 of the U.S.

Department of Agriculture.

RECOMMENDATION B8: HAZARDS FROM ADJACENT SITES

LAND-USE PLANNERS AND BUILDING DESIGNERS SHOULD

CONSIDER THE HAZARDS ARISING FROM DAMAGE ON THE

SITE OR ADJACENT SITES IN A NATURAL DISASTER.

Failures of dams, standpipes, gas lines, other "life-lines," etc. can produce

direct hazards or losses of vital services. Debris from adjacent weaker

buildings can be dangerous in earthquakes or winds. Both secondary hazards

and loss of services involving disruption of life support systems must be

19
considered for the site involved.

RECOMMENDATION B9: HAZARDS DURING

CONSTRUCTION

THE EFFECTS OF WIND AND EARTHQUAKE LOADING

ON PARTIALLY COMPLETED STRUCTURES SHOULD BE

CONSIDERED IN ERECTION PROCEDURES.

Although the life safety hazard usually is low, probably the greatest incidence

of wind induced failures occurs in partially completed structures. For both

wind and earthquake, such failures provide a significant hazard to adjacent

structures. The response of these structures to wind effects should be established and provisions made for adequate bracing to prevent failure. Seismic

loading during construction also should be given more attention.

The life safety hazard and dollar damage associated with failures of building

furnishings and equipment have only recently been evaluated. Considerable

mitigation of disaster effects can be achieved using current information

.

RECOMMENDATION BIO: DESIGN OF FURNISHINGS AND

EQUIPMENT FOR DISASTER

CONDITIONS

THE DESIGN OF MECHANICAL AND ELECTRICAL

EQUIPMENT AND OFFICE FURNISHINGS TO WITHSTAND

EARTHQUAKE AND EXTREME WIND LOADS SHOULD BE

BASED ON PROVIDING ADEQUATE BRACING AND

CONNECTION TO THE SUPPORTING STRUCTURE. THE

LOCATION OF SUCH EQUIPMENT WITHIN THE STRUCTURE

SHOULD BE SELECTED TO MINIMIZE THE INDUCED

FORCES AND CONSEQUENCES OF FAILURE.

Many piping, equipment, and elevator failures result from failure of their

supports and connections. Conflicting requirements for vibration isolation

and adequate tie down provisions are discussed in the review article by Ayres

and Sun. Possible approaches to equipment analysis and design are also presented by Sharpe, Kost, and Lord. Several solutions are given based on a consideration of the dynamic nature of the forces produced in these elements.

Tendencies for damage can be reduced by properly locating equipment within

the structure, such as avoiding heavy equipment on top floors of tall buildings. Design standards for elevators and their supporting structures must be

developed to mitigate loss of these vital lifelines.

Consideration should be given to special hazards related to the non- pe rmanent

nature of housing units such as mobile homes.

RECOMMENDATION Bll : TIE DOWNS FOR MOBILE

HOMES

MOBILE HOMES SHOULD BE TIED DOWN TO RESIST

EARTHQUAKE AND WIND LOADINGS. TIE DOWNS TO

RESIST WIND LOADING SHOULD BE PROVIDED IN ACCORDANCE WITH REQUIREMENTS INCLUDED IN THE

DCPA PUBLICATION TR 75 "PROTECTING MOBILE

HOMES FROM HIGH WINDS . "

20
Mobile homes and certain other non-permanent structures are subjected to

displacement from strong wind conditions and earthquakes. Design procedures

and criteria should provide for adequate support, anchorage and stability of

these units. While mobile home construction standards have been developed

and disseminated through ANSI A-119.1, they appear neither to have been

accepted nor implemented by the industry at large nor do they seem adequate

for hurricane wind frequently encountered in many sections of the United

States. The ANSI A-119.1 should be modified to include the 1972 provisions

on wind loading of ANSI A-58.1. Tie down requirements for wind loading

included in the DCPA publication appear to be the best practical procedure

which can be implemented at this time for existing units. These provisions

may be used as guidelines to develop requirements for earthquake loadings.

Actions by Standards Writers

Design loadings giving a better measure of disaster hazards should be

developed by standards writing bodies on the basis of up to date research

knowle dge

.

RECOMMENDATION B12: UPDATING SEISMIC CODES

MODEL CODE PROVISIONS AND COMMENTARY FOR

SEISMIC DESIGN SHOULD BE PREPARED ON A TOP

PRIORITY BASIS TO BRING THE MINIMUM LEVEL OF

PRACTICE INTO LINE WITH CURRENT STATE OF

KNOWLEDGE AND ANALYTICAL TECHNIQUES. THE

OBJECTIVES OF THE MODEL CODE PROVISIONS

SHOULD BE TO MINIMIZE LOSS OF LIFE AND PROPERTY DAMAGE AND TO MAINTAIN VITAL FUNCTIONS.

THE CODE PROVISIONS SHOULD BE WRITTEN SO THAT

THEY CAN BE APPLIED TO ALL AREAS OF THE UNITED

STATES . The last major revision in seismic code provisions occurred in the early

1960's. Since that time there have been significant advances in knowledge

of the response of building systems to seismic ground motion and correspondingly in analytical techniques and design procedures.

The development of the model code provisions should be undertaken by a small

qualified group with the guidance and advice of a broad-based professional

group. During the development of the code provisions, major interim recom- mendations should arise which can be incorporated in current practice.

RECOMMENDATION B13: RECURRENCE INTERVAL

DESIGN LOADING INTENSITIES SHOULD BE SPECIFICALLY RELATED TO RECURRENCE INTERVAL.

Consistent treatment of loadings occurring singly or in combination, requires

consideration of their recurrence interval. Although improvements in scientific knowledge may lead to future modifications in probability-based risk maps,

the best available information should be presented to designers in formats

suitable for use in decision making. Available evidence suggests that tornado

wind forces are feasible to design for, with the possible exception of winds

near the funnels of the largest tornadoes.

21
RECOMMENDATION B14: SITE EFFECTS

RELIABLE STANDARD APPROACHES SHOULD BE

DEVELOPED TO ACCOUNT FOR LOCAL SITE EFFECTS

ON THE INTENSITIES OF EARTHQUAKES AND WIND

LOADINGS

.

The forces induced by earthquake ground shaking have been observed to be

influenced by the natural frequencies of vibration of the building and surficial soil at the site. Donovan suggests approaches to such standards in

his review article. Wind loadings in ANSI A58.1 1972 account for site roughness effects but there is need to better define shielding and channeling

effects of topography and adjacent buildings. Power spectra for turbulence

need better definition for boundary layers representative of open country and

urban conditions, and for storm conditions including tornadoes, hurricanes,

thunderstorms, and extra- tropi cal storms.

RECOMMENDATION Bl 5 : LOADING DURATION

LOADING STANDARDS FOR DYNAMIC LOADS SUCH AS

EARTHQUAKES, WINDS, AND EXTREME LOCAL LOADS

SHOULD DEFINE LOADING DURATION.

Gust factors in the ANSI A58.1 standard relate design wind intensity to the

gust duration to which an element is sensitive. Improved statistical treatment of wind velocity as a function of duration would use a base averaging

period of 10 to 30 minutes in place of the current fastest mile of wind. The

damage potential of an earthquake depends markedly on the duration of strong

shaking; expressions of duration should accompany measures of loading intensity

such as acceleration, spectral velocity, and spectral displacement.

RECOMMENDATION B16: ELEMENT CHARACTERISTICS

STANDARD PROCEDURES SHOULD BE DEVELOPED TO

RELATE DESIGN LOADINGS TO THE CHARACTERISTICS

OF THE LOADED ELEMENT.

Research has shown that wind forces on individual elements such as roof

panels are markedly different from the forces affecting the main lateral

force system of the building. Similarly, earthquake- i nduced forces on mechanical and electrical equipment and furnishings differ substantially from

those induced in the lateral force system of the building. Potential payoffs

are large because these elements are responsible for large parts of both

building costs and disaster losses.

RECOMMENDATION B17: DEBRIS LOADS

STANDARDS FOR DEBRIS LOADINGS SHOULD BE MADE

AVAILABLE TO DESIGNERS.

Standard procedures are needed to define debris loading as a function of

location and site conditions. Impacts of airborne debris are responsible for

much of the wind damage to building exteriors. Debris loading may be a major

factor in progressive collapse. Waterborne debris from storm surges or flooding may produce impact damage or increased drag loading by impeding flow

of water

.

22
RECOMMENDATION B18: FIRE HAZARDS

FIRE HAZARDS ARISING FROM WIND STORM AND

EARTHQUAKE DAMAGE SHOULD BE DEFINED.

Substantial property damage is considered acceptable under extreme earthquake

and wind loadings (recurrence intervals exceeding about 100 years). The

philosophy is to accept property loss but prevent structural collapse leading

to severe hazards to life. However, damage to services and furnishings tend

to increase fire hazards; risks of life loss in fire must be defined as a function of damage to structure services and furnishings and the height of the

structure to allow consistent life safety provisions (it is easier to evacuate

a one-story house than a sixty-story office building). Major fires following

earthquakes must be considered a real possibility.

Analytical models and procedures are used in design to predict the response

of buildings to extreme loads. Standards are needed to guide designers and

regulatory officials in the selection and use of analytical techniques .

RECOMMENDATION 819: ANALYTICAL MODELLING

STANDARD PRACTICES SHOULD BE DEFINED FOR

ANALYTICAL MODELLING OF THE DYNAMIC CHARACTERISTICS OF BUILDINGS AND FOR ANALYTICAL

PREDICTION OF BUILDING RESPONSE TO EARTHQUAKES

AND EXTREME WINDS. ANALYSES, WHETHER INVOLVING

EQUIVALENT STATIC FORCES OR DYNAMIC FORCES,

SHOULD ACCOUNT FOR LATERAL, VERTICAL AND

TORSIONAL COMPONENTS OF LOAD AS WELL AS

STRUCTURE-FOUNDATION-SOIL INTERACTION, DAMPING AS A FUNCTION OF EXPECTED DEFORMATIONS,

PARTICIPATION OF NONSTRUCTURAL ELEMENTS,

OVERTURNING AND INSTABILITY EFFECTS.

Analytical methods are available for prediction of dynamic structural response

including effects of soi 1 -structure interaction and repeated and reversed

inelastic deformations. However, designers need guidance on reliable approaches which account for mass, stiffness, damping, cracking, reversed and repeated

inelastic deformation, the gravity loads working through lateral deflections,

and soi 1 -structure interactions. Results of laboratory model and field test

experience can confirm analytical models. Parameter studies can verify the

range of applicability of approximations in analysis. The torsional effects

of wind and earthquake loadings and the cross-wind response of tall, slender

buildings merit fuller investigation. The changes in stiffness and resistance

which result from repeated and reversed inelastic deformation should be

accounted for in analyzing earthquake effects on unsymmetrical or unconventional buildings.

RECOMMENDATION B20: MODEL STUDIES

STANDARD PROCEDURES SHOULD BE DEVELOPED FOR

MODEL STUDIES OF THE RESPONSE OF BUILDINGS

TO DYNAMIC LOADS.

Model tests in wind tunnels are widely used in design of important buildings

for wind effects. However, there is great need for calibrating different wind

tunnel test procedures through tests of a standard model and by comparison

with full-scale effects. Since the turbulent wind structure appears to be

dependent on wind velocity, the full-scale test must measure wind forces generated by the extreme design wind storm. Model tests under simulated earth23
quake loadings are beginning as shaking tables become available. Requirements

for standardized earthquake model test procedures, derived by comparison to

full-scale results, exist for tests of both structural components and elements

of the furnishings, mechanical, and electrical systems.

Planning and design criteria provide the des igner and regulatory official

with minimum standards for the design loadings , analytical procedures , and

the strength and ducti lity to be provided in the design. Improvements giving

more explicit indication of required performance and technically better

standards promote mitigation of dis aster losses and also economies in initial

cost where current standards are unduly restrictive.

RECOMMENDATION B21 : EXPLICIT LEVELS OF

PERFORMANCE

PLANNING AND DESIGN CRITERIA SHOULD STATE

EXPLICIT LEVELS OF DESIRED PERFORMANCE IN TERMS OF LIFE SAFETY, PROTECTION OF PROPERTY,

AND MAINTENANCE OF VITAL FUNCTIONS. PRESCRIPTIVE CRITERIA, CALIBRATED TO PROVIDE THIS

PERFORMANCE, SHOULD BE GIVEN FOR ORDINARY

TYPES AND MATERIALS OF DESIGNED CONSTRUCTION.

STANDARD DETAILS, ASSURING THE REQUIRED LEVELS

OF PERFORMANCE, SHOULD BE PROVIDED FOR SMALL

BUILDINGS FOR WHICH PROFESSIONAL DESIGN

ATTENTION IS NOT REQUIRED.

In the interest of simplifying the legal provisions of building codes,

achieving applicability in all areas of the United States, and at the same

time providing for acceptance of improved practices, efforts should be made

to move in the direction of performance criteria for the strength and ser- viceability of buildings under earthquake and wind loads. Performance

criteria can be formulated in terms of the recurrence interval of loading

and reliability of resistance (including stiffness, strength, and ductility)

desired for each limit state or class of limit states. Recurrence interval

of loading and reliability should be consistent with the consequences of

failure, that is, be greater for limit states involving hazard to life than

for those causing property loss or interruption of function. It is recognized

that substantial effort is required to define acceptable levels of performance

by direct consideration of total life cycle costs including social factors, or

by calibration with accepted good practice.

Prescriptive design criteria will continue to be used for well documented

types and materials of construction. However, as performance calibration of

prescriptive design criteria become better defined, codes will not need to

express these detailed design standards.

Acceptable details will continue to be required for some one and two story

buildings which can be accepted safely and economically without professional

participation in design.

There is a national need for a new comprehensive lateral force code. Present

code-referenced standards using equivalent static loadings and alternative

approaches using some form of dynamic analysis should be reviewed and evaluated. The more effective approaches for widely used types of structures and

materials of construction should be developed into design standards suitable

for code reference.

24
RECOMMENDATION B22 : LOAD FACTORS

CONSISTENT LOAD FACTORS SHOULD BE ESTABLISHED

FOR THE COMBINED CONDITIONS OF LOADING TO BE

CONSIDERED IN DESIGN, AND THESE SHOULD BE

APPLIED UNIFORMLY TO ALL TYPES AND MATERIALS

OF CONSTRUCTION. VARIABILITY FACTORS PROVIDING

CONSISTENT RELIABILITY IN STRENGTH AND SERVICEABILITY SHOULD BE ESTABLISHED FOR THE RESISTANCE

OF VARIOUS MATERIALS, COMBINATIONS OF MATERIALS,

AND TYPES OF CONSTRUCTION.

Load factors for combinations of dead, live, snow, wind, earthquake, etc.

loads should reflect consistently the probability of occurrence of the com- bination. For instance, all factored load combinations for strength might

define an intensity of loading with one specific recurrence interval. The

factors expressing variability of resistance should account consistently for

the uncertainties in resistance associated with the various types of construction and materials. It is recognized that this recommendation requires

modification of existing specifications, developed independently for different

materials, which have variously adjusted local factors and variability factors

to achieve the desired overall reliability. However, the recommended uniformity

provides simplicity in code language, eases the acceptance of improved practice,

and makes the level of risk in specific designs more evident to decision makers.

RECOMMENDATION B23: RELIABILITY OF DYNAMIC

ANALYSES

THE USE OF DYNAMIC MATHEMATICAL OR MODEL ANALYSES

IN DESIGN FOR EARTHQUAKE AND WIND LOADS SHOULD BE

ENCOURAGED BY ALLOWING ECONOMIES I N PROPORT I ON I NG

CONSISTENT WITH THE INCREASED RELIABILITY OF THE

DESIGN. GUIDELINES SHOULD BE DEVELOPED FOR

REQUIRING DYNAMIC ANALYSES FOR THOSE BUILDINGS

WITH HIGH IMPORTANCE, OF UNCONVENTIONAL TYPE, WITH

HIGHLY IRREGULAR SHAPES OR DISTRIBUTIONS OF RESISTANCE AND STIFFNESS, OR WITH FUNDAMENTAL NATURAL

PERIODS IN EXCESS OF A LIMIT ON THE ORDER OF ONE

SECOND .

One of a variety of equivalent static or dynamic methods of analysis may be

used in the design of a particular building provided that its idealizations

are dealt with consistently throughout the design process. Codes can credit

more rigorous analytical procedures by reflecting the greater reliability of

the design in the factor of safety required. A careful review for each type

of building and material is needed to define conditions where dynamic analyses

should be required.

RECOMMENDATION B24: DRIFT LIMITATIONS

LIMITATIONS ON STORY DRIFT SHOULD BE ESTABLISHED

TO REDUCE TO ACCEPTABLE LEVELS THE HAZARDS TO

PROPERTY DAMAGE AND STRUCTURAL COLLAPSE.

Structural response, which may be damaging to the structural and non-structural

elements or may threaten collapse of the structure, may be controlled by drift

limitations appropriate to the type of building, material, and level of re- sponse. Avoidance of human discomfort, which would also require consideration

of dynamic properties in expression of drift limitations, is not considered an objective under disaster conditions. Drift limitations to avoid damage to

building elements would consider elastic response and a load recurrence

25

494-610 0-73-3
interval for a moderate earthquake or moderately extreme wind. Drift limitations to avoid collapse would account for inelastic behavior including

available ductility and the destabilizing influence of vertical loads acting

through lateral deflections (P-delta effect) for a loading recurrence interval

corresponding to a major earthquake or extreme wind.

RECOMMENDATION B25 : MANUALS OF PRACTICE

MANUALS OF PRACTICE FOR BUILDING DESIGN

PROFESSIONALS SHOULD BE PREPARED TO DEMONSTRATE

APPROACHES TO MEETING PERFORMANCE CRITERIA AND

PRESCRIPTIVE REQUIREMENTS FOR WIDELY USED TYPES

OF BUILDINGS AND MATERIALS. MANUALS OF ACCEPTABLE DETAILS AND PRACTICES SHOULD BE PREPARED

FOR BUILDINGS WHICH DO NOT RECEIVE PROFESSIONAL

ATTENTION IN DESIGN.

Rational consideration of dynamic hazards such as earthquakes and extreme

winds requires building designers to work with concepts of dynamic response

and ultimate range inelastic behavior which are presently familiar to only

a minority of the profession. Manuals of practice will substantially aid

designers and regulatory officials in applying improved building practices.

Manuals of acceptable details and practices, for small buildings with low

functional importance which may safely and economically be constructed without

professional inputs, should demonstrate and explain the building qualities

required for successful performance and define the limitations of their applicability. These manuals must include a commentary explaining the philosophy

and goals used in their preparation to insure they do not become counterproducti ve "cookbooks . "

RECOMMENDATION B26 : CONTINUING EDUCATION

CONTINUING EDUCATION PROGRAMS SHOULD BE

DEVELOPED AND IMPLEMENTED FOR DESIGN PROFESSIONALS, REGULATORY OFFICIALS, AND BUILDERS TO

TRANSMIT THE KNOWLEDGE AND SKILLS REQUIRED TO

IMPLEMENT IMPROVED BUILDING PRACTICES FOR

HAZARD MITIGATION.

New concepts, information and techniques related to knowledge of extreme

loads, dynamic and inelastic structural behavior, and reliability and performance-based design approaches can be transmitted effectively to design

professionals through continuing education programs taught by knowledgeable

design professionals and professional educators. In this way, the improved

practices also will flow automatically into professional curricula. Comparable programs, using knowledgeable educators with practical experience

from technology programs of junior colleges, can train draftsmen, inspectors

and building tradesmen in improved practices for detailed design and construction.

Development of improved practices for survey and evaluation of existing

bui Idings can provide effective mitigation of losses from future natural

disasters and aid recovery from the ravages of disasters

.

RECOMMENDATION B27: METHODOLOGIES FOR

BUILDING SURVEYS

IMPROVED METHODOLOGIES SHOULD BE DEVELOPED FOR

ASSESSING THE SAFETY OF EXISTING BUILDINGS.

26
ACCOUNT SHOULD BE GIVEN TO HAZARDS OF POTENTIAL

EARTHQUAKES, EXTREME WINDS AND OTHER DYNAMIC

HAZARDS AS WELL AS THE EFFECTS OF DAMAGES

PRODUCED BY PREVIOUS EXTREME LOADS OR GRADUAL

DETERIORATION.

There presently are no nationally recognized effective, systematic and

economical procedures for assessing the hazards of existing buildings for

future extreme loads. Present design codes are not directly indicative of

the hazard represented by non-conforming existing buildings. Buildings which

do not conform to codes may either still meet the performance level intended

by the code or may be judged to represent acceptable risks when the very high

costs of abating hazards in existing buildings is considered. Appropriate

evaluation methods should be developed for systematic predisaster surveys of

safety for long-term use, and for emergency surveys immediately following a disaster for safety in emergency use. Evaluation procedures should receive

broad professional consensus. Criteria for acceptance or abatement of hazards

should be capable of reflecting the responsible authorities' assessment of the

social and economic consequences of action.

RECOMMENDATION B28: MANUAL FOR STRENGTHENING

AND REPAIR

A MANUAL OF PRACTICE SHOULD BE DEVELOPED FOR

THE STRENGTHENING OF EXISTING BUILDINGS AND

REPAIR OF DAMAGED BUILDINGS. LABORATORY AND

FIELD TESTS SHOULD BE CONDUCTED TO DOCUMENT

THE EFFICACY OF THE PRACTICES.

The documentation and professional discussion of strengthening and repair

procedures has been extremely limited. These procedures are usually expensive

to carry out and inadequate measures may have severe consequencies . Thus

benefits can arise from more effective procedures and reduced costs for

strengthening or repair.

RECOMMENDATION B29 : DATA GATHERING

PROCEDURES

STANDARD DATA GATHERING PROCEDURES SHOULD BE

DEVELOPED AND IMPLEMENTED FOR BOTH PRE-DISASTER

AND POST-DISASTER SURVEYS.

The development of rational strategies for disaster mitigation and relief

requires a nationwide inventory of disaster hazards. Efficient standard

procedures for data acquisition are an essential step in this effort. Postdisaster damage surveys can be efficient and effective for the many organizations needing damage information only if survey procedures are developed

in advance, personnel are trained in their use, and materials needed by

survey teams are stockpiled.

The same technical measures applicable to building structures are involved

in mitigation of losses of life, property , and function from failures of

furnishings and services . However , major recommendations are separately

stated here to bring them to the attention of the designers and author ities

responsible for these systems

.

27
RECOMMENDATION B30: FORCES ON EQUIPMENT

IMPROVED METHODS SHOULD BE DEVELOPED FOR

DEFINING THE LATERAL, VERTICAL, AND TORSIONAL

FORCES OR DISPLACEMENTS TO WHICH EQUIPMENT IS SUBJECTED IN EARTHQUAKES OR EXTREME WINDS.

When forces are defined, the level of resistance required for the desired

reliability can be determined by the procedures used in structural design.

Special attention to equipment forces or displacements is required because

the mass and flexibility of an item of equipment can cause its response to

differ from that of the portion of the building to which it is attached.

Equivalent static forces or displacements can sometimes be used for conventional equipment in conventional buildings. Dynamic analysis methods will be

needed in other situations as indicated in the review article by Sharpe, Kost,

and Lord.

RECOMMENDATION 831: MANUALS FOR EQUIPMENT

MANUALS OF PRACTICE SHOULD BE DEVELOPED FOR

SUPPORT, BRACING AND JOINTING DETAILS

AFFECTING THE SAFETY OF ARCHITECTURAL,

MECHANICAL, AND ELECTRICAL ELEMENTS AND

FURNISHINGS OF BUILDINGS IN NATURAL DISASTERS.

EDUCATIONAL PROGRAMS EMPLOYING THESE MANUALS

SHOULD BE PRESENTED TO THE PROFESSIONS AND

TRADES CONCERNED WITH THE DESIGN AND INSTALLA- . TION OF THESE ELEMENTS.

The life hazards and direct and secondary costs of damage to nonstructural

systems in natural disasters make these measures as important as the similar

ones recommended for the structural system. Shake table tests should be

required for details to assure improved performance.

RECOMMENDATION B32: CRITERIA FOR UTILITIES

AND INDUSTRIES

CODES AND STANDARDS AUTHORITIES RESPONSIBLE

FOR UTILITIES AND INDUSTRIAL FACILITIES SHOULD

DEVELOP AND IMPLEMENT CRITERIA ASSURING THE

PUBLIC SAFETY IN THE EVENT OF NATURAL DISASTER.

Building codes and standards often do not apply to utility and industrial

facilities where failure could cause substantial public losses of life,

property, and normal functions. Examples are water storage in dams and

tanks, chemicals such as ammonia and chlorine, petroleum products, and

electrical power transmission. Codes and standards should be developed to

provide consistent public protection from earthquake and extreme wind failures

of new facilities. Owners and regulators of existing facilities should review

hazards from existing facilities and take appropriate action in abatement.

Actions by Researchers

Additional knowledge of the environments which threaten disaster is needed

for more effective building practices . Continued research efforts can substantially improve capabilities to mitigate the hazards to buildings

.

28
RECOMMENDATION B33: EARTHQUAKE RISK MAPS

SUBSTANTIAL SE I SMOLOG I CAL RESEARCH ACTIVITIES

SHOULD BE FOCUSED ON SCIENTIFIC BASES FOR

REGIONAL EARTHQUAKE RISK MAPS.

These maps should describe the motions on firm ground as a function of recurrence interval, in a manner which permits geologists and engineers to

evaluate the hazards of failures of surficial soil deposits, buildings and

other manmade facilities. Studies of the types of focal mechanisms, the

statistical nature of earthquake occurrences, and the propagation of strong

motions in rock will provide fundamental knowledge for use in risk mapping.

Field and theoretical studies of active and potentially active faults related

to the location, extent, type, mechanics and regional tectonic patterns should

be expanded. Historical studies of seismicity in all areas of the U.S. and in

sei smol ogi cal ly similar areas abroad will increase the data base and, conse- quently, the reliability of risk assessments. The deployment of sei smol ogi cal

instruments throughout the U.S. should be reviewed and augmented to assure adequate identification of the occurrence of small magnitude earthquakes (to

aid in prediction of the recurrence interval of large earthquakes). The

deployment of strong motion instruments should be reviewed and augmented to assure adequate collection of information with consideration of differences

and uncertainties i n' an ti ci pated ground motions in various regions.

RECOMMENDATION B34: SITE EFFECTS IN EARTHQUAKES

SUBSTANTIAL GEOLOGICAL RESEARCH ACTIVITIES

SHOULD BE FOCUSED ON THE MECHANISMS BY WHICH

SURFICIAL DEPOSITS AFFECT THE INTENSITY AND

PHASING OF EARTHQUAKE GROUND MOTIONS, AS WELL

AS ON THE PROBABILITIES OF SURFACE FAULTING,

SUBSIDENCE, L I QU I FACT I ON , AND SLIDING.

Direct evidence of the effects of overburden on earthquake motions can be

obtained by placement of two and three dimensional arrays of strong motion

instruments in appropriate geologic and geographic areas. Material properties of the soil and rock profiles should be measured at all strong-motion

acce le rograph stations (unless this information is needed to select the siting

of the station or would seriously delay utilization of the recorded data, it can be collected after the occurrence of a significant motion). A program of

balanced installation costs and geotechnical investigation costs should be

developed to increase the number and widen the geographical distribution of

these stations.

RECOMMENDATION B35 : INSTRUMENTED BUILDINGS

THE NETWORK OF INSTRUMENTED BUILDINGS SHOULD

BE REVIEWED AND UPDATED TO ASSURE ACQUISITION

OF MOST NEEDED DATA.

Gaps in the capability for collecting and evaluating data on the response of

buildings to earthquakes can be minimized by tabulating building locations,

types of buildings and foundation, surficial geology, soil conditions and

detailed instrument locations for all buildings containing strong motion

accel erographs . Installations of time-history and peak reading instruments

should be considered for buildings exposed to strong winds. However, no building should be instrumented unless the construction drawings of the

building can be included in the installation data file.

29
RECOMMENDATION B36 : NEAR SURFACE

METEOROLOGY

MICROMETEOROLOGICAL STUDIES SHOULD BE CONDUCTED

TO DEFINE THE NEAR SURFACE CHARACTERISTICS OF

WINDS IN HURRICANES, THUNDERSTORMS, EXTRA- TROP I C AL

STORMS AND TORNADOES.

Existing data on the temporal and spatial variations of wind speed with height

and average wind velocity should be evaluated to define the additional information needed to predict wind loads on buildings and building elements.

Research should be implemented to fill gaps in the knowledge.

RECOMMENDATION 837: CL IMATOLOG I CAL ATLAS

CLIMATOLOGICAL DATA PERTINENT TO THE NEEDS OF

THE BUILDING COMMUNITY SHOULD BE IDENTIFIED AND

MADE AVAILABLE IN A CLIMATOLOGICAL ATLAS.

The atlas should account for localized effects of terrain features, principal

wind directions, etc. Wind environments related to serviceability (comfort

around buildings in normal weather, heating and cooling loads, etc.) may be

covered as well as hazards to life and property. Data collection programs

should be instituted to collect unavailable significant data.

Improved analytical methods are needed both as research tools to improve

understanding of the behavior of buildings under extreme loads, and as aids

to designers . The former class would be quite rigorous and complex , the

latter may be simpler and more efficient than current procedures

.

RECOMMENDATION B38: ULTIMATE DYNAMIC

RESPONSE

ANALYTICAL PROCEDURES SHOULD BE DEVELOPED TO

PREDICT IN DETAIL THE BEHAVIOR TO COLLAPSE

OF BUILDINGS UNDER DYNAMIC LOADS.

Salient factors include the dynamic and stochastic nature of the loadings;

three-dimensional interaction of lateral, vertical and torsional deformations

in diaphragms, shear walls, frames and nonstructural elements; and repeated

or cyclic inelastic deformations with degradation in stiffness and resistance.

All these features are unlikely to be efficiently represented in any one computer program for all types of buildings. However, the parameter studies

required for improvement of design practices for any type of building will

require consideration of all or most of these effects.

RECOMMENDATION B39: SIMPLIFIED ANALYSES FOR

DESIGN

SIMPLIFIED ANALYTICAL PROCEDURES SHOULD BE DEVELOPED FOR CODES AND GUIDES TO DESIGN.

The simplified procedures would be specific to limited classes of loadings,

buildings and materials. The range of validity should be studied carefully

and defined for users. These procedures should reflect the dynamic lateral,

vertical, torsional and overturning motions, and the amount of damping and

ductility available. Simplified procedures, rationally focused on salient

characteristics of structural behavior, can provide more insight for design

judgments than more rigorous complex and abstract methods.

30
The development of economical practices for buildings which will be safe

under extreme loads requires understanding the structural behavior (the way

forces and deformations distribute through the structure and the conditions

and mechanisms by which it may fail) . Understanding of behavior and confirmation of design concepts can come about only through experience . Although

structural tests are expensive, experience is achieved at substantially lower

social and economic cost if it occurs through deliberate testing rather than

by failures of bui Idings in natural disasters.

RECOMMENDATION B40: LABORATORY AND FIELD

EXPERIMENTS

LABORATORY AND FULL-SCALE FIELD EXPERIMENTS

SHOULD BE CONDUCTED TO INVESTIGATE ULTIMATE

STRUCTURAL BEHAVIOR UNDER LOADING CONDITIONS

REPRESENTATIVE OF EARTHQUAKES, EXTREME WINDS

AND SIMILAR HAZARDS. THESE TESTS SHOULD

INVESTIGATE ANALYSIS, DESIGN, STRENGTHENING

AND REPAIR PRACTICES AND DOCUMENT ABILITIES

TO PREDICT OR ACHIEVE SPECIFIC LEVELS OF

STRUCTURAL SAFETY.

A substantial part of the experimental effort would involve small specimens,

for instance to study loading rate effects on unit strength properties, and

static testing, for instance, to study effects of cyclic inelastic deformations on the resistance function. Also, there is great need for dynamic

large-scale testing to confirm or to develop understanding of the dynamic

response of buildings. Cooperative efforts should be explored with the

Atomic Energy Commission to determine whether studies of structural behavior,

soi 1 -structure interaction, and soil amplification and attenuation effects

can be conducted at the Nevada Test Site in conjunction with the underground

nuclear test program. A national earthquake engineering experimentation

facility should be established for testing large structures (about 100 ft.

square in plan) to failure under simulated earthquake motions with three

degrees of freedom.

RECOMMENDATION B41 : FIELD AND WIND TUNNEL

TESTS

LONG-TERM MEASUREMENTS SHOULD BE CONDUCTED ON

ACTUAL BUILDINGS IN STRONG WINDS TO ESTABLISH

DISTRIBUTIONS OF EXTREME LOADS, EFFECTS OF

REPEATED LOADS, AND DAMPING PROPERTIES. COORDINATED WIND TUNNEL STUDIES SHOULD BE CONDUCTED

TO DEVELOP MODELLING TECHNIQUES FOR TURBULENT

FLOW CONDITIONS AND AEROELASTIC INTERACTIONS.

The field measurement program should emphasize minimal instrumentation of

many structures, rather than elaborate and costly instrumentation of a few,

to achieve strong response information at reasonable cost. Measured dynamic

response would be statistically correlated with wind speed and pressure data

and results of dynamic analyses. Wind tunnel modelling techniques would be

critically reviewed through correlations with field measurements to define

modelling procedures reliable for design.

RECOMMENDATION B42: ROCK AND SOIL MATERIALS

LABORATORY AND FIELD STUDIES OF THE BEHAVIOR OF

ROCK AND SOIL MATERIALS UNDER LARGE DYNAMIC

DEFORMATIONS SHOULD BE CONTINUED TO ASSIST IN

31
DETERMINATION OF THE RELATIONSHIP BETWEEN BEDROCK

AND SURFACE MOTIONS IN EARTHQUAKES.

The non-linear behavior of soils and rocks causes the bedrock to surface

relationships for strong motions to differ from those measured in the field

for low intensity explosive or seismic excitations. The non-homogeneity of

in-situ soils and rocks leads to substantial differences from the relationships that would be derived from dynamic laboratory tests of small samples.

A thorough investigation is needed to develop and confirm effective and

economical approaches to predicting effects of surficial deposits on earthquake ground motions.

Analytical , experimental , and field research studies are needed to provide

knowledge for improved design criteria.

RECOMMENDATION B43: MECHANISMS OF FAILURE

LABORATORY STUDIES OF THE MECHANISMS FOR FAILURE

OF STRUCTURAL ELEMENTS AND CONNECTIONS, UNDER

CYCLIC OR REPEATED INELASTIC DEFORMATIONS

REPRESENTATIVE OF THE EFFECTS OF SEVERE DYNAMIC

LOADS, SHOULD BE CONDUCTED TO DEFINE STIFFNESS,

STRENGTH, DAMPING, DUCTILITY, AND SERVICE LIFE.

The development of design provisions which assure intended performance of

structural elements and connections is impossible without detailed understanding of their behavior. Current information consists only of a few pilot

studies under cyclic inelastic deformations, some information on loading rate

and strain history effects for coupons under simple states of stress, and

substantial, but incomplete, information for structural elements and connections under monotonic loading.

RECOMMENDATION 844: DAMPING

INFORMATION ON DAMPING IN BUILDING STRUCTURES

SHOULD BE ASSEMBLED FROM ALL AVAILABLE FOREIGN

AND DOMESTIC SOURCES, ANALYZED, AND AN EXPERIMENTAL PROGRAM INITIATED TO FILL IMPORTANT GAPS.

Damping is a vital parameter governing structural response to dynamic loads

in both serviceability and ultimate ranges. The amplitude of the motions

under which damping was measured must be known to define whether the results

pertain to service or ultimate behavior. The prior loading history of the

structure in which the damping was measured must be known to determine effects

of accumulated damage on damping.

RECOMMENDATION B45: STRENGTH AND DUCTILITY

REQUIREMENTS

STOCHASTIC ANALYTICAL PARAMETER STUDIES SHOULD

BE CONDUCTED TO INVESTIGATE THE REQUIREMENTS

FOR DISTRIBUTION OF STRENGTH AND DUCTILITY IN PRINCIPAL TYPES OF STRUCTURES FOR EXTREME

DYNAMIC LOADS SUCH AS EARTHQUAKES AND WINDS.

SYNTHESES OF THESE STUDIES SHOULD BE PREPARED

IN THE FORM OF PRACTICAL ANALYTICAL AND DESIGN

APPROACHES FOR DETERMINING AND PROVIDING THE

REQUIRED DISTRIBUTIONS OF STRENGTH AND DUCTILITY.

32
Research in analytical methods and structural behavior provides a capability

to predict effects of earthquakes, winds, and other dynamic hazards. However,

the ultimate objective is better buildings. Expert studies of the experimental

evidence, with rigorous analytical studies used to enlarge the scope of parameters considered, will permit synthesis of practical design procedures and

criteria which exploit effectively the new research knowledge.

RECOMMENDATION B46 : NON-STRUCTURAL ELEMENTS

A MULTI-DISCIPLINARY PROGRAM OF ANALYTICAL,

EXPERIMENTAL, AND DESIGN STUDIES SHOULD BE

CONDUCTED TO ACQUIRE KNOWLEDGE AND DEVELOP

STANDARDS FOR IMPROVING PRACTICES OF DESIGN

FOR NON-STRUCTURAL BUILDING ELEMENTS.

Earthquake experiences have shown such extensive life hazards and property

losses from failures of furnishings, mechanical systems and service systems

that priorities are difficult to define. Functioning elevators are vital to

evacuations of tall buildings prior to secondary hazards such as fire.

Emergency services such as power, water and communications must be preserved.

The resistance of ceilings, shelving, lighting fixtures and partitions must

be improved to reduce life hazards and property losses.

33
ACKNOWLEDGMENTS

The success of the workshop was due to the enthusiastic participation of

all attendees. However, special contributions by several individuals should

be acknowledged.

Charles Thiel of the National Science Foundation and Ugo Morelli of the

Office of Emergency Preparedness assisted in formulating the cooperative

Federal program leading to the workshop. The Applied Technology Council of

the Structural Engineers Association of California arranged for five of the

workshop papers and their review by the Seismology Committee of SEAOC. The

workshop program and arrangements were coordinated by H. S. Lew. Staff

members from the National Bureau of Standards Boulder Laboratory assisted

in conducting the workshop and supplying the support services and required

facilities. Secretarial support for the workshop was provided by Sharon

Weeks and Lynn Androsik. The complete workshop proceedings were typed with

great care by Barbara Mullinix.

34
APPENDIX A

WORKSHOP PRQGRAfI
APPENDIX A

iJBS/jjSF liATIQlJAL WORKSHOP Qli BUILDING PRACTICES

FOR

DISASTER FlITIbATIOii

PROGRAM

MONDAY, AUGUST 28. 1972

8:30 A.M. Registration

9:00 A.M. Welcoming Address and Introductory Remarks

-- H. S. Boyne, Chief

Quantum Electronics Division

NBS/Boul der

-- J. R. Wright, Director

Center for Building Technology, NBS

-- R. N. Wright

Deputy Director (Technical)

Center for Building Technology, NBS

-- C. Thiel, Program Manager

Division of Advanced Technical Applications, NSF

MORNING SESSION CHAIRMAN - R. N. Wright

Deputy Director (Technical)

Center for Building Technology, NBS

9:30 A.M. VALUES AND COSTS

H. Kunreuther, Economist

University of Pennsylvania

10:15A.M. BREAK

10:30 A.M. APPROACHES TO IMPLEMENTATION

P. Baseler, Codes Consultant

House Springs, Missouri

11:15 A.M. EARTHQUAKE HAZARDS FOR BUILDINGS

N. Donovan, Foundation Engineer

Dames and Moore

11:30 A.M. THE PROBLEM OF SEISMIC ZONING

S. T. Algermissen, Director

Seismic Research Group

National Oceanic and Atmospheric Administration

12:15 P.M. LUNCH

36
AFTERNOON SESSION CHAIRMAN - C. Culver

Disaster Research Coordinator

Office of Federal Building Technology

Center for Building Technology, NBS

1:30 P.M. HIND HAZARDS FOR BUILDINGS

J. Vellozzi, Associate and Civil Engineer

J. Healey, Structural Engineer

Ammann and Whitney

2:15 P.M. ABNORMAL LOADING ON BUILDINGS AND PROGRESSIVE COLLAPSE

N . Somes , Chief

Structures Section

Center for Building Technology, NBS

3:00 P.M. BREAK

3:15 P.M. LAND USE PLANNING AND NATURAL DISASTER MITIGATION

J. Wiggins, President

W. Petak, Vice President

M. McCoy, Research Associate

D . Moran , Consul tant

J. H. Wiggins Company

J. Slosson, Geologist

James E. Slosson and Associates

W. Monash, Planning Director

Santa Cruz Company, California

4:00 P.M. ARCHITECTURAL APPROACHES TO HAZARD MITIGATION

E . Hi 1 1 man , Pre si dent

Hillman, Biddison and Loevenguth

A. Mann, Senior Vice President

Daniel, Mann, Johnson and Mendenhall

4:45 P.M. ADJOURNMENT

6:00 P.M. RECEPTION

TUESDAY, AUGUST 29, 1972

MORNING SESSION CHAIRMAN - S. Kramer, Chief

Office of Federal Building Technology

Center for Building Technology, NBS

9:00 A.M. PROCEDURES AND CRITERIA FOR EARTHQUAKE RESISTANT

DESIGN (PART I) C. Pinkham, President

S. B. Barnes and Associates

9:15 A.M. PROCEDURES AND CRITERIA FOR EARTHQUAKE RESISTANT

DESIGN (PART II)

N . Newmark , Head

Department of Civil Engineering

W. Hall, Professor, Civil Engineering

University of Illinois

37
1:00 A.M. PROCEDURES AND CRITERIA FOR WIND RESISTANT DESIGN

J~! Vellozzi, Associate and Civil Engi neer

J. Healey, Structural Engineer

Ammann and Whitney

):45 A.M. BREAK

:00 A.M. CRITERIA FOR BUILDING SERVICES AND FURNISHINGS

J . Ayers .Principal

T. Sun, Vice President

Ayers, Cohen, and Hayakowa

I : 45 A.M. LUNCH

AFTERNOON SESSION CHAIRMAN - C. Thiel

Program Manager

Division of Advanced Technology Applications, NSF

1:15 P.M. BEHAVIOR OF STRUCTURAL ELEMENTS

B. Bresler, Professor

University of California

2:00 P.M. BEHAVIOR OF STRUCTURAL SYSTEMS _ UNDER DYNAMIC LOADS

R. Sharpe, Executive Vice President - Operations

G. Kost, Assistant Vice President

John A. Blume and Associates

J. Lord, Director of Systems Engineering

Albert C. Martin and Associates

2:15 P.M. BREAK

3:00 P.M. SURVEY AND EVALUATION OF EXISTING BUILDINGS

F. McClure, Partner

McClure and Messinger

3:45 P.M. ADJOURNMENT

EVENING FREE

WEDNESDAY. AUGUST 30, 1972

9:00 A.M. Meetings of the Following Subcommittees:

and SUBCOMMITTEE #1 - "Implementation of Earthquake Hazard

Reduction Practice"

1 : 30 P.M.

SUBCOMMITTEE #2 - "Formulation of Earthquake Hazard

and Reduction Practice"

7:30 P.M. SUBCOMMITTEE #3 - "Implementation of Wind Hazard Reduction

Practi ce

SUBCOMMITTEE #4 - "Formulation of Wind Reduction Practice"

38
THURSDAY. AUGUST 31 , 1972

MEETINGS OF SUBCOMMITTEES #1-4

Exchange Comments on First Draft of Recommendations

Between Subcommittees and Rework Accordingly.

FRIDAY . SEPTEMBER 1 , 1972

SESSION CHAIRMAN - S. Kramer, Chief

Office of Federal Building Technology

Center for Building Technology, NBS

9:00 A.M. Group Discussion of Recommendations

1:00 P.M. ADJOURNMENT

39
APPENDIX B

REVIEW ARTICLES

40
VALUES AND COSTS

by

Howard Kunreuther*

I . I ntroducti on During the past two years the United States has had more than its normal

share of damage from the natural elements: an earthshaking event in San

Fernando, a windblown affair in Lubbock, Texas and most recently a wall of

water invading Rapid City and a stormy Agnes creating havoc in the Northeast.

Not only have these recent disasters demonstrated that nature knows no boundaries but they have underlined the importance of conferences such as this one

on disaster mitigation and damage prevention.

One of the lessons to be learned from these recent events is the failure

of communities and individuals to adequately protect themselves against

potential damage from disasters. In San Fernando a number of buildings

including a hospital were poorly designed and did not withstand the force of

the quake. Few individuals living in the area carried earthquake insurance

even though the rates were relatively modest. For example, the premium rate

for wood frame houses is twenty cents per $100 of coverage for straight

earthquake insurance and fifteen cents per $100 if the risk is endorsed on a homeowners policy; both are subject to a mandatory five percent deductible.

(Federal Insurance Administration [19], p. 14.) Even though Rapid City was

one of two communities in South Dakota that qualified for the federal government's subsidized National Flood Insurance, I'ew families had taken out

policies. An official of St. Paul Fire and Marine Insurance Company commented

that despite newspaper ads and other promotions we tried this year, we were

only able to sell 300 policies covering $367,000 in two years [20].

Since few individuals are willing to protect themselves voluntarily

against the consequences of natural disasters it may be necessary to develop

formal guidelines for better building practices in the future. In making

decisions on what type of structures should be built in hazard prone areas,

detailed questions must be raised which I have categorized under four different headings :

1. Data on intensity and frequency of the hazard

a. What data do we have on the probability of a particular type

disaster affecting a particular area (e.g., an earthquake of

intensity VIII in Bakersf iel d)

?

b. How sensitive are final decisions to changes in these

frequencies? In other words, is accuracy of probabilistic

estimates critical in choosing one type of structure over another?

2. Cost and damage information

a. What are the differences in costs between types of construction?

b. What damages are likely to result to a given structure from a disaster of specified intensity?

Wharton School of Finance and Commerce, University of Pennsylvania.

41

494-610 O - 73 - 4
c. How sensitive are final decisions to changes in these damage

figures over time?

3. Factors difficult to quantify

a. How do you calculate the value of a human life and/or the cost

of severe injury?

b. What differences exist between the private and social costs

of human losses?

c. How sensitive are final decisions on building practices to

changes in these values?

Whoshouldbearthecostsofdisasters?

a. Individuals living in the hazard-prone areas

b. Federal government

c. Joint private and public responsibility.

Fortunately a number of excellent recent studies by engineers, geologists, seismologists and others have provided us with statistical estimates

on expected damage from earthquakes and disasters caused by high winds.

McClure [11] has supplemented these methodological reports by providing

detailed estimates of replacement costs of buildings damaged by the 1952

Bakersfield earthquake. By developing an approach for analyzing this

information we can determine the acceptable level of risk which should be

tolerated from a specific disaster. Not only will this acceptable risk

level be a function of the accuracy of the statistical data, but it will

also be critically dependent on the answer to the question: Who should bear

the cost of natural disasters? As we shall see, the optimal type of structure in a given area may differ if future disasters are viewed from the eyes

of a prospective homeowner in the area or an individual taxpayer in another

part of the country.

To illustrate the critical elements in evaluating alternative building

practices in hazard-prone areas, I will next consider a specific example

which involves the choice of building a brick masonry structure or reinforced

brick structure in the fictitious town (I hope) of Shakerville, Missouri.

The example will be intentionally oversimplified to illustrate concepts and

hence can be criticized from the point of view of accuracy. I look forward

to learning more about the specifics on frequency data and costs from individuals at the workshop far more knowledgeable in this general area than I am

.

II. Deci s i on-Maki ng in the Private Sector

A. Damage and Probability Estimates

Consider a family who is building for the first time in Shakerville or relocating in the area after having its home totally destroyed by a severe earthquake. The following steps would be required to specify the expected

annual damage to a particular structure from earthquakes:

1. Determine the frequency of any earthquake of a given magnitude

to the particular area in question. Techniques for doing this

For an excellent discussion of a methodology for gathering earthquake

damage statistics see Coast and Geodetic Survey [18].

42
have been detailed by Blume [2], Housner [5], and Coast and

Geodetic Survey [18].

2. Compute the intensity of ground shaking over a given area for

any specific quake. Housner [5] suggests that the distribution

of intensity over a given region can be depicted by contour lines

with the greatest intensity near the fault and diminishing intensity

at an increasing distance from the fault.

3. Combining figures on the frequency it is then possible to compute

the probability that a given site will experience ground motion

which will be of a certain intensity.

4. For each particular structure it is possible to compute the

expected damage if the particular structure receives ground shaking

of at least a certain intensity. A detailed damage survey for

Bakersfield has been undertaken by McClure [11] and has been elaborated in Coast and Geodetic Survey [18].

For simplicity assume that the family is considering building either a brick

masonry structure or a reinforced brick home. Table 1 presents the following

illustrative comparative data on costs and potential damage from an earthquake. As you can see, we have greatly simplified the problem by assuming

that there would be no damage from any quake with intensity less than or equal to VI; the only two threatening events are quake of intensity VII and

VIII with respective annual probabilities of occurrence of .02 and .01.

Reinforced brick structures are safer than brick masonry buildings as shown

by the expected annual damage figures in Table 1 but they cost slightly more

to build. For example, a $50,000 brick masonry house would cost approximately

$53,000 if the walls were made of reinforced brick.

Table 1 EXPECTED ANNUAL PHYSICAL DAMAGE FROM AN EARTHQUAKE FOR A BRICK MASONRY AND REINFORCED BRICK STRUCTURE

Intensity of Earthquake

(Modified Mercalli

Seal e

)

Annual Probabi 1 i ty

of

Occurrence

Expected Annual Damage

to Structure j (percent of Original Cost)

(i)

Brick

Masonry

Rei nf orced

Brick

VI or less .97 0 0

VII .02 10 0

VIII .01 60 20

This cost differential was obtained in the following manner. Reinforced

brick masonry costs approximately 60 per cent more per square foot than brick

masonry (i-e-» $3.60 instead of $2.25). The building walls account for

approximately 10 per cent of the total cost of a structure. Thus the walls

of a $50,000 brick masonry house would cost $5,000. The identical reinforced

brick structure would then cost approximately $3,000 more or $53,000. I would like to express my appreciation to Alan Yorkdale, Director of Engineering and Research at the Structural Clay Products Institute for providing me with these cost differentials. For a more detailed discussion of cost

estimates for different type building walls, see [17].

43
B . Cost Comparisons for the Two Structures

A number of tangible and intangible cost elements will determine the

homeowner's decision as to which house he will purchase. We will consider

each of the factors in turn and then determine the sensitivity of this final

decision to changes in his estimates.

B.l Mortgage Terms

Undoubtedly the most tangible element with respect to the homeowner's

final decision are the terms of his mortgage. Both the length of the mortgage, the rate of interest and the downpayment requirements will influence

the homeowner's choice. Table 2 shows how the annual payments per $1,000

loan are affected by the interest rate and the length of the mortgage. An

increase in the interest rate and/or a decrease in the length of the loan

will require higher payments on the part of the borrower and will thus favor

the purchase of the cheaper house. If all other factors remain the same, we would thus expect a homeowner to favor the brick masonry house over the

reinforced brick home when money is tight and hence interest rates are higher.

Thus a 30-year loan at an annual rate of 5 per cent will require annual payments of approximately $65 per $1,000 in contrast to $117 per $1,000 for a 20-year loan at 10 per cent.

Table 2 EFFECT OF INTEREST RATE AND LENGTH OF LOAN

ON ANNUAL PAYMENTS PER $1,000 LOAN

Length of Loan

^\(in years)

Annual v,,^ Interest

Rate (in %) 20 25 30

5 80 .24 70.95 65 .05

6 87.18 78.23 72 .65

7 94.39 85.81 80.59

8 101 .85 93.68 88.83

9 109.55 101 .81 97.34

10 1 1 7.46 110.17 1 06. 08

B.2 Expected Cost of Physical Damage

Who should bear the cost of damage to property from natural disasters?

Currently there is a liberal system of disaster relief subsidized by the

federal government where a combination of grants and low-interest loans are provided to individuals who suffer home or business losses. At the other

extreme, we would require the homeowner to pay for his own losses by taking

out a loan at the market rate of interest if he did not have sufficient

insurance protection. The decision on the type of structure which should be

erected and its location will be greatly influenced by who bears the cost

after a disaster. We will illustrate differences which may result by

44
contrasting a self-insurance policy where the individual must bear the losses

himself with the current federally subsidized relief policy.

a. Self-Insurance Policy . To determine the expected annual cost of a self-insurance policy, it is necessary to specify both probabilistic and

damage figures as we have done in Table 1. In more general terms, suppose

there are m possible events (e.g., types of weather phenomena that can affect

a given community) with each event i having an annual probability of occurrence, p^. . Suppose each event can cause damage D:^ to a given type structure

j in the community, where D^. may equal zero for a number of possible events.

For our example, suppose event i is a quake of intensity VII so that p^. = .02.

If the structure j is a $50,000 brick masonry house, then (from Table 1)

D'j = $5 ,000 (i.e., 10 per cent of $50 ,000); if structure j is a $53 ,000

reinforced brick structure, then D'^ = 0. The expected annual damage from a disaster for a specific structure j is given by

. m . E(dJ) = Z p.D^.

i=l ^

^ If the homeowner were to take out a loan at the market rate of interest

whenever he received any damage to structure j then his expected annual costs

in present value terms would be E(Dj).

b. Current System of Disaster Relief . The cost of an earthquake to the

private homeowner or businessman suffering damage will be quite different if

the current system of federal relief prevails. A brief summary on the

changing role of the federal government with respect to disaster relief

should provide us with insight into how the present policy evolved.

At the turn of the century, people who were located in hazard-prone

areas had to bear almost the entire risk of damage from natural disasters

themselves. Relatively little insurance coverage was available and aid to

the private sector was limited to voluntary charitable contributions. Since

1900, there have been several important developments which have reduced the

losses which a person has had to bear himself, thus encouraging more people

to locate in these areas than would otherwise have been the case. Red Cross

contributions were the first formal source of relief to individuals suffering

damage from a disaster. This organization, established by a federal statute

in 1905 has had a long and distinguished history of private charity. Raising

all of its funds through private donations, the Red Cross uses them to provide relief to victims suffering disaster losses.

Beginning in 1953, the federal government assumed a direct responsibility for financially aiding individuals hurt by a disaster. In that year the

Small Business Act was passed, authorizing the SBA to offer low-interest

loans to homeowners and businesses suffering injury from natural disasters.

The general purpose of SBA disaster loans is "to restore a victim's home or business property as nearly as possible to its pre-disaster conditions."

Before the Alaskan earthquake, the agency provided 3 per cent loans with a maximum repayment period of 20 years to cover the exact amount of physical

damage. It was understood that the borrower would use the entire loan

strictly for the purpose of rebuilding or repair. The severity of the damage

in Alaska caused concern that unless the SBA liberalized its policy many

individuals would not qualify for a disaster loan because of their inability

to pay off their old mortgages and other debts and still make monthly payments to the SBA.

Perhaps the most significant revision of SBA policy was the authorization of loans for substantial debt retirement for any homeowner or business

45
suffering losses from the earthquake. He was given funds not only to repair

his damaged structure, but also to retire old debts (for example, outstanding

mortgages, accounts payable) which may have had nothing to do with the

disaster itself. Thus, instead of continuing to pay conventional 7 or 8 per

cent rates on these outstanding claims, the borrower could now retire them at

a subsidized 3 per cent rate. A further reduction in the size of a victim's

monthly payment was achieved by permitting a 30-year amortization period

instead of the normal 20-year maturity of SBA loans. If the property owner requested it, the agency would waive any principal and interest during the

first year of the loan and on principal up to an additional four years. Thus,

the victim's burden was minimized; in fact, in a number of cases, particularly

for businesses, the borrower was financially sounder after the disaster than

before the "catastrophe."

Although the SBA made it very clear that its actions in Alaska were

taken to meet a special situation, there is clear evidence that the agency

has not retreated to its more stringent policy. Using the Alaskan case as a precedent, a Congressional bill was passed at the end of June 1965, authorizing the SBA to permanently extend its maximum load period from 20 to 30 years.

The Southeast Hurricane Disaster Relief Act of 1965 authorized the Small

Business Administration to "forgive" a part of each loan up to maximum of

$1,800, a provision which was not even permitted in Alaska.^

The Disaster Relief Act of 1970 increased the forgiveness amount to

$2,500 and permitted the annual interest rate on loans to be set up to two

per cent below the rate for 10-12 year government securities but never higher

than 6 per cent per year. A recent Congressional Act (PL 92-385) inspired by

the Rapid City floods and Hurricane Agnes has liberalized SBA policy even further. An owner of a home or business which receives damage from a disaster

occurring between January 1, 1972 and July 1, 1973 can obtain a forgiveness

grant of up to $5,000 and a 30 year loan at one per cent per year to cover

the remaining losses.

To see the effect of the federally subsidized loans on private costs,

suppose the average annual damage to a given structure from an earthquake is

$1,000.^ Assume that the owner can take advantage of a 30-year disaster loan

at an interest rate below the market rate. Table 3 computes the present

value of this loan for variations in both the SBA and market rates of interest. Naturally, if the SBA rate and market rate are identical the present

value to the homeowner of a $1,000 loan will be $1,000. On the other hand,

if the SBA rate is 1 per cent and the market rate is 7 per cent, the present

value of the loan drops to $481.^

In general, as the market rate of interest increases and/or the SBA

interest rate decreases, the present value of the loan also decreases. Hence

even though the expected annual losses of a brick masonry building are con- siderably higher than an identical reinforced brick structure, the loss differentials between the two in present value terms is narrowed considerably

by virtue of the SBA disaster relief policy.

For more details of the equity and efficiency of the SBA disaster loan

policy, see Kunreuther [9], and Dacy and Kunreuther [4], Chapters 9 and 10.

There would, of course, be long stretches of time where the building would

not receive any damage; however, if a quake occurred, the damage might be

$100,000. On the average, an annual loan of $1,000 would be required to

repair the structure.

The figures in Table 3 do not take into account the forgiveness feature of

the SBA Policy and hence understate the benefits of PL 92-385 to the

individual.

46
Table 3 PRESENT VALUE OF 30-YEAR $1 ,000 LOAN FOR COMBINATIONS OF SBA

AND MARKET ANNUAL RATES OF INTEREST

^^""^""".v..,^ Market Interest

^\Rate (in %)

SBA Interest^^-v,.^

Rate (in %) 5 6 7 8 9 10

1 596 533 481 436 398 365

2 686 61 5 554 503 459 421

3 784 702 633 574 524 481

4 889 796 718 651 594 545

5 1 ,000 895 807 732 668 613

B.3 Intangible Costs from a Disaster

Up until this point in the analysis, we have dealt with physical losses

from a disaster. When one introduces intangible factors, such as the value

of a human life, more difficult estimation problems arise. At the outset,

we should distinguish between the private cost perceived by the individual and

the public cost borne by society. If an individual is killed by a disaster

then the costs to society include not only the expenses of training and educating this person, but also his potential earning stream discounted back to

the present. Normally, an individual does not look backwards or project

forward in this manner when making his decisions and so may implicitly value

his life less (in dollar terms) than society would. ^ He may thus take actions

which would be justified when using his own sets of values but would be con- sidered unwise from the perspective of the general public.

Recently, attempts have been made to estimate the value of a human

life.^ By studying "loss of life" lawsuits one finds that jury awards have

ranged anywhere from $50,000 to $500,000. These two extreme values could

provide an upper and lower bound to the value of a human life. An alternative

method would be to estimate this value by considering an individual's potential future earnings. Suppose a person's annual salary is estimated to be

$10,000 for each of the next 25 years; then his discounted future earnings

(at a 7 per cent annual interest rate) would be $116,636, which would be a proxy for the value of his life. Finally, an FAA study to compute the value

of life saving in commercial air transport accidents estimated that the

indirect and direct costs yielded a life value of $373,000 per average

f atal i ty .

The value of a human life is only part of the story. We must also

consider the probability that one or more individuals will be in a building

destroyed by a particular disaster. For example, the probability may be

quite high that if a warehouse is damaged by an earthquake, any individual

in the building at the time will be killed. But the percentage of time that

For a more detailed discussion of these points, see Starr [16].

For an interesting description of these studies, see Otway [14].

47
any one person is likely to be in the warehouse is quite small. In other

words, we must also determine the occupancy factor associated with a particular structure. The occupancy factor has been defined by Blume [2] to be

the ratio of the total person occupancy-hours per year to the theoretical

number of hours per year that the building would be occupied if it was filled

to capacity.-^ For example, if a residence houses a family of four (i.e.,

capacity is four) then the theoretical total number of hours per day that

the house could be occupied would be 4(24) = 96 hours. In practice, suppose

only 48 person-hours would be occupied on a typical day in the year. Then

the annual occupancy factor will be .5. The number of person-years of

exposure (N) for a family of four is simply (4)(.5) = 2. An increase in

either the occupancy factor and/or capacity will increase the expected number

of lives lost from a given disaster. For ease of illustration we will assume

that all individuals in a house are killed if it collapses from a quake;

otherwise there are no loss of lives. For each event i (i.e., a quake of a given intensity) there is some probability q^ that structure j will collapse.

Since the probability of this quake occurring has been already specified to

be p^ the expected annual number of lives lost for structure j is the simply

. m . E(lJ) = N Z p. q^

i = l ^

^

where N = number of person-years of exposure.

These data are assembled in Table 4 for the brick and reinforced brick

homes when N = 2. The assumed probability of collapse (q:j) to either structure is relatively small even for an earthquake of intensity VIII. As

expected, the reinforced brick home is considerably safer than a brick

masonry structure.

Table 4 EXPECTED NUMBER OF LIVES LOST PER YEAR FROM AN EARTHQUAKE

PER BRICK MASONRY AND REINFORCED BRICK STRUCTURE

(Based on Number of Person-Years of Exposure N = 2)

Intensity of

Earthquake

(Modi f i ed

Mercalli Scale)

Annual

Probabi 1 i ty

of

Occurrence

Annual Probability that

Structure Collapses

Given Occurrence

of Quake (q;?)

Expected Number of

Lives Lost Per Year

From Quake i

(N p. q^)

(i)

Brick

Mason ry

Rei nforced

Brick

Brick

Masonry

Reinforced

Brick

VI or less .97 0 0 0 0

VII .02 .01 0 .0004 0

VIII .01 .02 .01 .0004 .0002

Estimates of occupancy factors for particular buildings appear in Blume

[2].

48
B.4 Evaluating the Two Structures

Table 5 summarizes the factors which will determine the type of structure to build and presents illustrative figures for brick masonry and reinforced brick homes. We have assumed that the homeowner is considering

building either a $50,000 brick masonry home or the identical reinforced

brick house for $53,000. A downpayment of $10,000 would be made for either

house and the balance would be taken out as a 25-year mortgage at the market

rate of interest (7 per cent per year). The expected annual physical damage

figures are based on the data from Table 1 and the expected number of lives

lost is taken from Table 4. Using these figures we can easily compute the

expected annual total cost for each structure under a self-insurance policy

and under the current disaster relief policy.

Table 5 FACTORS DETERMINING CHOICE OF STRUCTURE TO BUILD AND ILLUSTRATIVE

FIGURES FOR BRICK MASONRY AND REINFORCED BRICK HOMES

Factor

Cost of Structure

Downpayment

Outstanding Mortgage

(1) Length of Loan

(2) Annual Rate of Interest

Annual Mortgage Payments

Expected Annual Physical

Damage [E(Dj)]

Present Value of Loan for E(D~^):

Self-Insurance Policy*'

Federal Disaster Relief

Policy^

Expected Annual Lives Lost ECL"^)

Type of Structure

Brick Masonry Reinforced Brick

$50 ,000 $53 ,000

$10 ,000

40 ,000

25 years

7%

$1 0 ,000

43 ,000

25 years

7%

$ 3 ,432'

400'

400

192'

0008

$ 3 ,690'

106'

106

51'

,0002

^Based on Table 2.

'^Based on figures from Table 1.

r Loan discounted at market rate of interest.

'^30-year loan at 1 per cent annual interest rate.

^Figures are based on Table 3 with 7 per cent market rate of interest.

^Based on figures from Table 4.

49
Based on present value analysis, Table 6 shows that the brick masonry

structure will be chosen if the proposed disaster relief bill is enacted and

the value per human life is slightly less than $200,000. If a self-insurance

policy is followed, the reinforced brick house will be more desirable, no matter what value is placed on a human 1 i f e

.

Table 6 COMPARISON OF EXPECTED ANNUAL COSTS FOR BRICK MASONRY AND

REINFORCED BRICK STRUCTURES FOR DIFFERENT

VALUES PER HUMAN LIFE

(Based on figures from Table 5)

Type of Rel i ef Pol i cy

Value Per Human Life Sel f-Insurance Current Disaster Relief

Brick

Mas on ry

Reinforced

Brick

Brick

Masonry

Rei nf orced

Brick

$100 ,000 $3,912 $3 ,816 $3 ,704 $3,761

200 ,000 3 ,992 3 ,836 3,7 84 3 ,781

500 ,000 4,232 3,896 4,024 3 ,841

How sensitive are these decisions to changes in cost estimates? We

have already indicated that an increase in the interest rate (and consequently the discount rate) will have two effects: it will increase the differential in monthly payments between the brick and reinforced homes and it will

decrease the expected annual physical damage differential between brick

masonry and reinforced brick structures if disaster relief is federally subsidized. Both of these changes will make the brick masonry house a more attractive buy. On the other hand, an increase in the value of a human life

will push the decision in favor of the reinforced brick home.

A graphical illustration of the type of sensitivity analysis which can

be undertaken is depicted in Figure 1, based on data from Table 5, for both

a self-insurance policy and the federal relief program. Suppose the expected

annual physical damage differential of a brick masonry house over the identical reinforced brick home was $200; looking at Figure 1 it is clear that even

if the value of a human life was as low as $96,333 the reinforced brick home

would be preferred under a sel f -i nsurance policy. If a federally subsidized

disaster relief policy was in effect then Figure 1 shows that the value of a human life would have to increase to $270,000 before the reinforced brick

home would be preferred.

It should be pointed out that we are making the implicit assumption in this

analysis that the individual is risk neutral so his utility function is

linear in money. If a person has some aversion to risk, then he will

attempt to avoid large losses and may want to build the reinforced brick

house even when present value analysis suggests that the brick masonry home

is less expensive. If, on the other hand, individuals tend to underestimate

the probability of extreme events, they may choose the brick house over the

reinforced structure even when the "objective" figures suggest the opposite

decision.

50
III. Deci si on-Maki ng for Public Facilities

Theoretically it is possibly to analyze the costs and values of public

facilities in the same way that we studied the private homeowner's decision.

Now the relevant actors in the drama are the entire community (rather than an individual homeowner) and the taxpaying public. Because public facilities

frequently affect the entire community and even the region, losses resulting

from a disaster may be considerable. One way to compare the economic value

of different public facilities is by introducing the concept of an importance

factor . The importance factor should measure both the expected direct disaster losses to a given type structure in a particular locality as well as the

indirect losses after a particular disaster if the building has lost its

functional capability. The direct losses can be captured by determining

expected annual physical damage as well as expected loss of life and/or

injuries to people in the structure. The indirect losses reflect the critical

functions each structure must perform during the immediate post-disaster period. To estimate this component one must consider alternative sources which

may temporarily replace the damaged facility during the emergency period. For

example, if there are a number of hospitals in a particular region then the

importance factor associated with a particular one will be much lower than if

it is the only facility serving the region. Similarly, regions which are centrally located would assign lower importance factors to structures than areas which are somewhat isolated. Table 7 presents a sample list of those

public facilities which perform critical functions during the immediate aftermath of a disaster and have relatively high importance factors.

Table 7 PUBLIC FACILITIES WITH CRITICAL FUNCTIONS

IN IMMEDIATE POST- D I SASTER PERIOD

Faci 1 i ty Critical Function

Fire Fighting Equipment

Police Department Facilities

Telephone Center

Hospi tal

s

Sewage Treatment Plant

Water Treatment Plant

Shelter Facilities

Food Supply Facilities

Roads and Highways

Airports, Train Stations and Ports

Mitigation of Physical Damage

Maintaining Law and Order

Commun i cations

Treating Injured Victims

Waste Disposal

Providing Purified Water

Temporary Housing

Feeding Residents

Transportation

Receiving Outside Food and Medical

Suppl ies

There will also be long-run economic effects associated with disasters

which should be included in the concept of an importance factor. The destruction of public facilities such as transportation and communication networks

can severely affect the recovery process and the productive capability of a community and hence have indirect regional and national effects. By studying

51
these long-run costs more closely it should be possible to develop a priority

list for rebuilding public facilities. For example, it may not be feasible

to reconstruct commercial buildings damaged by an earthquake until transportation facilities have been at least partially restored. The magnitude

of outside aid and the speed with which it is forthcoming will play an important role in determining the long-run economic impact of natural hazards

on the community. Despite the large amount of destruction caused by a disaster, recovery may be very rapid if capital in the form of low-interest

loans and grants is immediately forthcoming. In fact, the disaster may

actually turn out to be a blessing in disguise for the community. Aside from

the positive short-run economic effects triggered by the reconstruction

activity, there is an opportunity for improving damaged public and commercial

facilities. More research is needed on specifying the long-run impact of

natural hazards on a community and the positive opportunities for mitigating

future losses from hazards.^

The federal government provides more generous relief to repairing

damaged public facilities than to restoring damaged private property. In 1950 the first comprehensive Federal Disaster Act (PL 81-875) was passed

which provided generous federal disaster assistance with respect to restoration of local facilities if the President declared the region "a major disaster area." In the ensuing years, a number of special bills have been

passed following specific disasters,^ culminating in the Disaster Relief Act

of 1970 (PL 91-606). This act authorized up to 100 per cent reimbursement

of the cost of permanent repair, restoration or replacement of disasterdamaged state and local public facilities, rather than just allowing only

emergency repairs or temporary replacement. Federal loans are contingent

upon compliance with applicable codes, specifications and standards.

Following the San Fernando earthquake which damaged or destroyed several

private medical care facilities. Congress amended PL 91-606 by authorizing

similar grants to tax-exempt nongovernment medical care facilities (PL 92-

909 ) . Consider a community decision with respect to location and type of

construction of a proposed hospital. From the above remarks, it is clear

that the community must balance the importance of this type of public

facility following a disaster with the generous public relief forthcoming if

the building is damaged or destroyed. The first factor argues for a very

safe building in a nonrisky area; the second factor works in the opposite

di recti on

.

Rather than being forced to attach specific dollar estimates to the

factors which are difficult to quantify, it is possible to undertake the same

type of sensitivity analysis for public facilities that led to Figure 1. It

is likely that for each factor there will be a wide range of dollar values

that imply the same optimal decision with respect to location and choice of

building material. As in the private sector, this range of dollar values

will be a function of the type of disaster relief program in effect. Under

the current system of disaster relief the federal government bears the entire

costs of reconstruction and the expenses associated with emergency relief.

Other things being equal, we would then expect a community to build more public structures in hazard prone areas than if the local residents were solely responsible for financing their own recovery.

Empirical evidence on the long-run recovery process following natural

disasters is presented in Dacy and Kunreuther [4], Chapter 8.

For a summary of disaster legislation, see OEP [12], Volume 1, pp. 167-173.

52
Expected Annual Physical

Damage Differential

Figure 1. Effect of damage differential, value per human life and type of disaster

relief policy on decision to build brick masonry or reinforced brick house.

53
IV. Alternative Cost Bearing Approaches

As we have already seen, the current federal relief policy treats

disasters as if they were a public responsibility. In other words, every

taxpayer in the U.S. bears a fraction of the costs of disaster-induced damage

to sections of the country. As a result of this policy, individuals may find

it in their own best interests to build less expensive but more disasterprone structures in high risk areas. A look at the type of structures and

their locations in California should convince us that this has actually

happened. Let us examine five alternative cost-bearing approaches to disaster rel ief

.

A . Total Federal Responsibility

If we accept the notion that disasters are "acts of God" which must be

borne by all members of society then we should have a disaster relief policy

whereby each homeowner who suffers a loss is given an outright grant to

rebuild his house. Furthermore he should have the freedom to construct it in

any part of the country he wishes without any restrictions. The New Zealand

government's current attitude toward earthquake damage illustrates this point

of view. Currently they have an earthquake and war damage insurance program

where all property insured against fire is automatically protected against

earthquake losses. The earthquake coverage is an extension of insurance

against war damage which was developed during the latter part of World War

II, at which time it was felt that the entire country should pay for any

property damage by the enemy. ^ An Earthquake and War Damage Commission has

since been established to determine government policy toward natural disasters. Although the Commission theoretically can require certain building

standards before issuing insurance coverage, they have not actually done this nor have they established any criteria for safe buildings. Hence, an individual has the freedom to locate his home and business in any part of the

country and will be protected against earthquake losses.

B . Sel f- Ins urance by Homeowner

Another extreme approach is the se 1 f -i nsurance method where each individual pays for the entire cost of the disaster. We are only beginning to

learn about people's perceptions of extreme events, but the evidence from

geographers and psychologists suggest that individuals employ numerous mechanisms to reduce uncertainty about these events so as to avoid dealing

with th em

.

Unfortunately, the owner of a structure is normally not made aware of

the risk of living in the area by the developers. Aside from considerations

of social responsibility, there is no built-in incentive for the contractor , architect, or engineer to develop safer but more costly buildings in hazard - jrone areas unless there are formal restrictions placed on them such "aT )ui1ding codes~ Al though the builder or contractor who constructs a poo r 1

y

designed house which is damaged by a disaster may be theoretically at fault

on a charge of negligence or misrepresentation, it may be very difficult for

the homeowner to win his case in court. ^ We are a long way from the days of

For discussion of the Hew Zealand program, see [18], Appendix A, pp. 86-87

and 0 ' Riordan [13]

.

For some specific illustrations of how individuals in hazard-prone areas perceive the risks of natural hazards, see Burton and Kates [3], Kates [8]

andSlovic, Kunreuther and White [15].

For an excellent detailed discussion of the risks and legal liability from

earthquakes, see Hughes [6].

54
Hammurabi where the contractor was put to death if the house he built

collapsed and caused the death of the homeowner. The victim today must rely

on charity or federal aid or renege on his outstanding debts.

A current example is Wilkes Barre, Pennsylvania where thousands of

homeless families without flood insurance are willing to declare bankruptcy

rather than paying off the mortgage on their flooded homes. As a result,

the President has asked Congress for an unprecedented $1.7 billion in relief

funds to aid these victims. If the past is any guide to the future, then

there is little chance that we will permit victims of disasters to suffer

the consequences of their lack of foresight. Hence, reliance on self-insurance as an effective mechanism for preventing future damage is less than

perfect. When the chips are down, we would most likely view relief to

victims as a public responsibility.

C . Required Insurance Protection

An alternative method of protection against disasters would be through

the mechanism of insurance.^ Rather than providing federal relief following

a disaster each individual would be required to have some form of insurance

protection against possible future losses from disasters. The insurance

requirement could be enforced directly through government fiat or indirectly

by banks to protect their outstanding mortgages. In either case, the individual would be required to pay a premium based on the expected annual costs

from a disaster.

If the premiums were based on risk then insurance would provide

information on ways that individuals could protect themselves against a disaster. Using the figures from Table 5, reinforced brick structures would

only have an annual premium of $2 per $1,000 (i.e., $1 06/$53 , 000 ) while brick

masonry structures would be charged $8 per $1,000 (i.e., $400/ $50 ,000 ) . Individuals would then have precise information on the expected annual damage to

each of these structures from natural hazards. After taking into account the

difference in their personal estimate of intangible costs (e.g., loss of life)

they could then determine whether the extra construction costs of reinforced

brick were more than offset by the reduced insurance premiums.

There are practical difficulties associated with utilizing insurance as

a mechanism for optimizing disaster preparedness. Not only would it be

costly to develop premiums which would differentiate between types of structures and location, but the complexity of the rate schedule would be very

confusing to the homeowner. There is also no easy way to make sure that the

homeowner has met the standards upon which his premium is based. There would

thus have to be a cost of checking reflected in the rate structure. If the

cost of determining the rate structure, transmitting the information to the consumer and inspecting the structure were incorporated in the rates then the

permium might be considerably higher than the actuarial figure. It might

then unnecessarily discourage some individuals and businesses from locating

in a particular area where it may be profitable for them to do so.

D . Land Use Restrictions and Building Codes

Given the transaction costs and imperfections associated with marketing

insurance it may be possible to achieve similar results through land use restrictions and building codes. ^ In theory, the most efficient way would

be to form a national commission of experts who could collect information on

For a critique of current disaster insurance programs, see OEP [12), Vol. 1,

pp. 135-145.

Specific land use and building code ordinances are discussed and critiqued

in OEP [12] , Vol . 1 , pp. 125-1 33.
disaster, analyze it to determine the marginal benefits of different types

of preventive activities and suggest a methodology for evaluating these

alternatives. The commission could disseminate information on the potential

costs of future disasters to homeowners and businesses while prescribing a set of building codes and zoning regulations. By centralizing the decisionmaking process, some of the inefficiencies of the market place could be

eliminated. As we have seen, there is little incentive for the individual

builders, homeowners or businesses to worry about future disaster problems

so mechanisms for enforcing building codes and land-use restrictions must

be developed.

The costs associated with enforcing building code restrictions and

land-use regulations may be more than balanced by benefits in the form of

reduced future disaster losses. In this sense these enforcement costs are

a type of public investment. Arrow and Lind [1] have shown that it is

appropriate to evaluate public investments (e.g., costs of enforcing alternative building codes) by simply comparing expected annual costs as we did

in our illustrative example. It should thus be relatively straightforward

to specify the appropriate building codes as a function of construction

costs, expected physical damage and intangible costs. If one is not certain

of the intangible costs, then a diagram such as Figure 1 can be utilized for

sensitivity analysis. For example, it may be possible to show that a building

code requiring a hospital to be of reinforced brick would be preferred over

any other material if the value of a human life was more than $20,000. It

would then only be necessary to specify whether a human life was worth more

than $20,000. If the answer was "yes" then reinforced brick could be prescribed, otherwise, one of the other materials would be more desirable.

Once nationally accepted land-use planning and construction standards

have been developed, initiative for enforcing and tailoring the regulations

to specific characteristics of the area may be more effectively handled at

the state or local level. An actual illustration of this point is the recent

adoption of a specific and restrictive building code by the City of Long

Beach to improve the ability of buildings to resist earthquake damage. The

new code criteria were developed by equating involuntary earthquake risk with

other voluntary risk situations, such as automobile accidents.^

E . A Suggested Program

Current federal policy suggests that the public feels some degree of

responsibility toward helping victims of natural disasters. The increasing

costs of these events also indicates that preventive action is justified from

an economic standpoint. The challenge lies in developing a policy which

strikes a balance between satisfying the objectives of the individual living

in a hazard-prone area and the general public.

The Federal Flood Insurance program may serve as a prototype for

developing a plan to mitigate future earthquake and windstorm losses.^

Specifically some form of comprehensive disaster insurance could be made

available to homes and businesses in a hazard-prone area but only after the

community had taken positive steps toward reducing potential losses by

enforcing adequate land use measures and building code regulations. The

initiative could thus lie with the communities rather than with the federal

government. In return, existing structures would be insured at a subsidized

insurance rate while new buildings would be charged an actuarial rate. In

For a more detailed description of the recommendations and nature of the

study undertaken for Long Beach, see Wiggins and Moran [21].

For a detailed description of the provisions in the National Flood Insurance

Act of 1968, see OEP [12], Vol. pp. 136-138. A complementary cost sharing

model for structural flood protection programs is discussed in Loughlin [10].

56
essence, the federal government would help pay the costs of protecting

individuals now residing in hazard-prone areas from future disaster losses

while requiring that the communities make these areas safer places in which

to live.

For such a plan to have any chance of success, the federal government

would have to withdraw its liberal disaster assistance programs such as the

SBA low-interest loans. As we have already seen, there would otherwise be

no financial incentive for communities to develop land-use measures or building codes. The most effective way of achieving a favorable reaction by

communities toward such a program would be for federal agencies, such as VA

and FHA, and private lending institutions to require some form of comprehensive disaster insurance as a condition for mortgage. The owner would then

want to follow better building practices with respect to his property simply

as a way of reducing his insurance premium. For such a system or some variant

of it to be successful, each of the concerned groups must recognize the need

to jointly defend themselves against an unpredictable woman--Mother Nature.

V. Summary and Conclusions

The following conclusions emerge from this survey of values and costs

of natural hazards

.

1. Rather than attaching specific dollar estimates to factors which

are difficult to quantify, some form of sensitivity analysis

should be undertaken. There may be a wide range of dollar values

implying the same optimal decision with respect to location and

choice of building materials.

2. More specific data is needed to determine the costs associated

with damage to public facilities. Community, regional and

national considerations must be included in the analysis.

3. The federal disaster relief policy plays a critical role in

private and public sector decisions with respect to location of

structures and type of construction. Currently the federal

government is bearing the lion's share of the costs of natural

di s as te rs .

4. A methodology for determining the appropriate site and types of

building materials for a given structure should be written in a form that the potential user can understand.

5. Insurance supplemented by land-use restrictions and building

codes appears to be an appropriate policy for shifting the

cost burdens of disasters from the general taxpayer to

individuals living in hazard-prone areas.

6. There must be more dialogue between the theoretician and the

practitioner as to ways of mitigating losses from future

disasters.

VI . References

Arrow, K. J. and Lind, R. C, "Uncertainty and the Evaluation of Public

Investment Decisions," American Economic Review , June 1970, 60, pp. 364-

78.

Blume, J. A., "Civil Structures and Earthquake Safety," in [7] pp. 109-

116.

57

494-610 O - 73 - 5
3. Burton, I. and Kates, R. W., "The Perception of Natural Hazards in

Resource Management," Natural Resources Journal , III (January 1 964)

pp. 412-441

.

4. Dacy, D. and Kunreuther, H., The Economics of Natural Disasters

(New York: The Free Press (MacMi llan) , 1969.

5. Housner, G. W., "Engineering Estimates of Ground Shaking and Maximum

Earthquake Magnitude," in Proceedings of the Fourth World Conference

on Earthquake Engineering , Santiago, Chile, 1969.

6. Hughes, T. H., "Risks of Earth Movement and Liability that Results

Therefrom," in [7] pp. 71-76.

7. Joint Committee on Seismic Safety to the California Legislature

Earthquake Risk Conference Proceedings (September 22-24, 1971).

8. Kates, R. W., Hazard and Choice Perception in Flood Plain Management

(Department of Geography Research Paper No. 98) (Chicago: Uni versi ty

of Chicago, 1965).

9. Kunreuther, H., "The Case for Comprehensive Disaster Insurance,"

Journal of Law and Economics , 11 (April 1968) pp. 133-163.

10. Loughlin, J. C, "A Flood Insurance Model for Sharing the Costs of

Flood Protection," Water Resources Research , VII (April 1971), pp.

236-244.

11. McClure, F. E., "Studies in Gathering Earthquake Damage Statistics,"

(Washington, D.C.: Coast and Geodetic Survey, 1967).

12. Office of Emergency Preparedness, Disaster Preparedness Vol. 1-3 , (Washington, D.C.: Office of Emergency Preparedness, January 1972).

13. O'Riordan, T., "The New Zealand Earthquake and War Damage Commission--

A Study of a National Natural Hazard Insurance Scheme," Natural Hazard

Research Working Paper No. 20 , 1971.

14. Otway, H. J., "The Quantification of Social Values," in [14a] pp. 1-13.

14a. Otway, H. J., (ed.). Risk vs. Benefit: Solution or Dream (Los Alamos,

1972).

15. Slovic, P., Kunreuther, H., and White, G., "Decision Processes,

Rationality and Adjustment to Natural Hazards," (mimeo), June 1972.

16. Starr, C, "Social Benefit vs Risk," in [7], pp. 4-14.

17. Structural Clay Products Institute, Comparative Ultimate Cost of

Building Walls , (McLean, Virginia: Structural Clay Products Institute,

1 972 ) .

18. U.S. Department of Commerce, Coast and Geodetic Survey, Studies in

Seismicity and Earthquake Damage Statistics, 1969 . A Report Prepared

for the Department of Housing and Urban Development, Office of Economic

and Market Analysis, 1969,

19. U.S. Department of Housing and Urban Development, Federal Insurance

Administration, Report on Earthquake Insurance to the Congress of the

Uni ted States , Washington, D.C., 1971.

20. Wall Street Journal, "Insurance Group Says Only $14 Million Insured

in Rapid City Floods," (June 15, 1972, p. 16).

58
21. Wiggins, J. H., Jr., and Moran, D. F., Earthquake Safety in the City of

Long Beach Based on the Concept of Balanced Risk , (mimeo) , 1971

.

ADDENDUM*

Undertaking Benefit-Cost Studies

Benefit-cost studies should provide valuable data inputs for developing

a disaster mitigation program. It may well not be economically feasible to

mitigate completely the effects of natural hazards, since the potential benefits may not justify the additional expenditures. It is necessary to have a solid base for deciding how much of our resources should be devoted to mitigating hazards as compared to other priorities, and for deciding which disaster

mitigation methods are most effective. Important studies suggested by the

discussions of the subcommittee are:

• Desirability of accelerated programs of replacing older buildings

especially susceptible to collapse during natural disasters. An

initial question might be: Are the costs of tearing down a building

and replacing it by another balanced by sufficient reduction in

expected losses from natural disasters. At a higher level, should

the Federal government invest money into tearing down old unsafe

structures or would it be more beneficial for them to allocate these

funds to subsidizing new construction?

• Desirability of restricting construction near faults or in other

hazardous zones. One question that must be answered is: Are the

needs of the community best served by imposing (and possibly paying

damages for securing) such land use restrictions?

Many factors must be considered when facing such questions, and systematic

analyses based upon adequate data are essential. Benefit-cost analysis also

will provide reliable data inputs upon which to base decisions with respect

to building code provisions, land use plans, and insurance premiums or other

loss indemnification programs.

Benefit-cost studies may be carried out on many different levels, and

the elements in the objective function and the measures of effectiveness will

vary accordingly. The paper by Kunreuther deals with an individual's decision

concerning a greater level of earthquake resistance in the construction of a new home. Here the concern is primarily to minimize the individual's total

annual dollar cost, and secondarily with the risk of loss of life in the

individual's family. However, when a decision involves the public welfare,

other costs--both tangible and i ntangi bl e--must be considered. Hence, in

developing a set of recommendations for a program of disaster mitigation,

the objectives from the individual, community and national point of view must

al 1 be consi dered.

In carrying out benefit-cost studies for disaster mitigation, various

viewpoints with respect to the role of the public and private sectors must

be kept in mind. Due to political pressures following a disaster, the

Federal government has borne, and probably will continue to bear, a substantial portion of the costs of damage to private facilities and all the

costs to public sector facilities. On the other hand, some feel that the owner of a building should be encouraged to protect himself against losses

from earthquakes through mechanisms such as insurance. The paper by Kunreuther

*This material was prepared by Howard Kunreuther and Robert Whitman during the

workshop committee sessions.

59
illustrates the effect of differing policies upon an individual's or community's response to measures that might reduce the hazards from earthquake .

The need for such studies is not new. This need was clearly recognized

and very well stated in recommendation A-7 of the Task Force on Earthquake

Hazard Reduction (Office of Science and Technology, 1970). The difficulties

in carrying out realistic studies are also well known and well documented

(Perspective on Benefit-Risk Decision Making, Committee on Public Engineering

Policy, National Academy of Engineering, Washington, D.C.). The following

discussion suggests a general approach for benefit-cost studies as applied to

the earthquake hazard problem, and identifies steps that must be taken to make

it possible to accomplish meaningful studies in the near future.

Framework for Specific Studies

In very simple terms, benefit-cost studies for earthquake hazard mitigation might involve the following steps:

1. Identify and inventory the physical units (residences, commercial

structures, public facilities, etc.) involved in the study.

2. Establish the probability that the physical units will be subjected

to different intensities of ground shaking.

3. Establish the probability that the physical units will experience

various levels of damage, as a function of the intensity of ground

shaking.

4. Determine the many different tangible and intangible costs associated

with the various levels of damage.

5. Establish the different tangible and intangible benefits derived

from alternative strategies for disaster mitigation.

These various bits of information are then combined and displayed in a manner

that shows the benefit-cost tradeoffs. The actual manipulation of the information is a straight forward procedure on today's computers; the problems

lie in assembling valid input information and in interpreting and using the

results. One point, which will be repeated later, is worth emphasizing here:

Attempts to assign dollar values to loss of life or other factors which are difficult to quantify should be delayed until the very final phase of any

study. In many cases, it will be possible to show that a particular decision

will be optimistic for a wide range of dollar values assigned to any one of

these factors.

Assembling Input

Obviously, the results of a benefit cost analysis can be no better than

the input. For a major analysis, assembling the basic input information can

be a major task. The steps involved in collecting input for earthquake

hazard studies are illustrated by Steinbrugge et al (1969) and Whitman et al (1972). Certain types of input collection efforts are fundamental to all

studies, and will be mentioned here.

1. Inventory of Physical Units : There is a need for an inventory and

data on all buildings that are located in disaster hazardous urban areas of

the United States. The magnitude of the work of gathering this data is so large that this information should first be gathered for buildings with high

socio-economic values - emergency centers, hospitals, and other occupancies

with higher disaster support responsibilities. The appropriate federal

60
agency should make a feasibility study of how to undertake such an inventory

which would provide data on the type of occupancy, type of construction and

other details of construction needed to make viable potential risk evaluations.

The magnitude of such an inventory could exceed the National Fallout Shelter

Survey Program and, therefore, a feasibility and pilot study is recommended

as a f i rst step

.

2. Seismic Risk : Adequate data concerning the probability of ground

shaking of various intensities is essential to all benefit-cost studies.

Such data should be compiled for all parts of the United States. Approaches

to assembling such information are described in the review articles by

Algermissen and Donovan, and specific recommendations for implementation of

these approaches appear elsewhere in this report. These recommendations

should be implemented in two steps: an initial study based on available

information using crude measures of the intensity of ground shaking, and a long-range program using more complete and more quantitative measures of

ground shaking. Funding should be provided as necessary to complete the

initial study by 1974.

3. Damage Probabilities : Nominally similar physical units will not

necessarily experience the same damage for a given intensity of ground

motion. For each type of physical unit, studies are necessary to express

the probability of damage as a function of intensity of ground shaking. Once

the physical unit{s) to be documented in the pilot inventory study have been

selected, a concurrent study of damage probabilities should be undertaken.

It is recommended that this concurrent study be accomplished via contracts

with engineering organizations experienced in earthquake engineering design

and in study of damage during past earthquakes.

4. Associ ated Costs : While the costs necessary to repair or replace

a damaged building are quantifiable, there potentially are many other costs

associated with the health, safety and welfare of individuals and with the resources of the community and nation. These associated costs are in part

short-run and in part long-run. Some are tangible and some are intangible

but all are difficult to quantify. Some may be expressed in dollars; others

are not easily expressible in dollars, and, at least in early parts of the

study, attempts to express them in dollars should be avoided.

It is necessary to distinguish between the private costs perceived by

the individual and the public cost borne by society. If an individual loses

his life from a disaster, then the costs to society include not only the

expenses of training and educating this person, but also his potential

earning stream discounted back to the present. Normally, an individual does

not consider all these factors and hence may implicitly value his life less

(in dollar terms) then society does. Benefit-cost analysis as viewed by an individual may suggest a different decision than when viewed from the point

of view of the community or the nation.

Data on the importance of a particular structure immediately following

a disaster must also be considered. The concept of an importance factor

should measure both the expected direct disaster losses to a given type

structure in a particular locality as well as the indirect losses after a particular disaster if the building has lost its functional capability.

Data on the long-run economic effects associated with disasters also

are essential. For example, the destruction of public facilities such as transportation and communication networks can severely affect the recovery

process and the productive capability of a community and hence have indirect

regional and national effects. Reconstruction priorities may be determined

if such data is made available.

It should be possible to develop associated cost data that will be

applicable to a wide spectrum of benefit-cost analysis. This will entail

61
considerable study and research, and the effort should be funded at a level

sufficient to ensure rapid progress.

Application of Results

A benefit-cost study is a useful tool for legislative bodies and executives. It also is a technique whereby a complex problem may be reduced to

terms comprehensible by the general public. The output of the analysis must

be in a form that permits comparison of different alternatives while enabling

the legislator, executive or layman to make his own judgments on the relative

importance of the different benefits and costs. As an alternative to attaching specific dollar estimates to factors that are difficult to quantify, some form of sensitivity analysis should be used. In this way, it may be possible

to show that one type of policy alternative will be preferred to another over

a wide range of dollar estimates.

References

Steinbrugge, K. V., F. E. McClure, and A. J. Snow (1969), "Studies in

Seismicity and Earthquake Damage Statistics, Appendix A", ESSA-Coast

and Geodetic Survey.

Whitman, R. V., Cornell, C. A., Vanmarke, E. H. and Reed, J. W. (1972).

"Optimum Seismic Protection and Building Damage Statistics." M. I. T Department of Civil Engineering Report R72-17, sponsored by National

Science Foundation.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation, Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards , August 28-September 1,

1972, Boulder, Colorado (Issued February 1973).

62
APPROACHES TO IMPLEMENTATION

by

Paul E. Baseler

Introducti on

The conclusion drawn from recent studies by the Office of Emergency

Preparedness that "Land-use and construction regulations containing strong

disaster mitigation features can in the long run alleviate losses caused by

natural disasters" [1] appears justified by investigation of current practices. Variations in requirements in Federal standards, state and municipal

building codes and in the several model codes, [2, 3, 4, 5] especially those

providing for design for resistance to earthquakes, and in criteria for the

application and enforcement practices for those requirements by local jurisdictions indicate need for greater accord than now exists.

It is not our purpose in this paper to explore the differences in the

technical requirements for protection from natural disasters. We are concerned here with the manner in which these requirements are formulated,

the basis for their application by Federal agencies and state and local

governments, and with the extent and effectiveness of their enforcement,

since this may result in differing degrees or levels of protection being

provided in the various jurisdictions.

Scope of Review

The term "natural disasters" covers a wide range of subjects: wind in

varying velocities; earthquakes; flooding as the result of rainfall; landslides, etc. Those conditions in which these forces are applied to buildings

or specific elements of construction are usually covered in building codes.

Land-use codes generally are used to cope with flooding and landslide conditions by prohibiting or limiting construction of buildings in areas subject

to such conditions. This division of coverage has influenced this study.

The typical approach to determining land-use regulations (planning and

zoning laws) and construction regulations (building and building service

systems codes) differs:

Planning and land-use requirements are primarily influenced by

local conditions. Suggested extent of coverage, recommended text

for regulation of various conditions, and criteria for applying

these are provided by state, regional, or professional organizations serving that field of local governments. But complete

ordinances suitable for adoption in local jurisdictions by

reference, perhaps with minor modifications, are not widely

available. Consequently planning practices and zoning laws are primarily products of local governments, usually developed by

professional consultants on the basis of conditions peculiar to

the community.

In contrast to this, building regulations are primarily technical

requirements based on accepted design criteria or specific

Paul E. Baseler, R. A. -Codes Consultant. Former Building Commissioner, City

of Jennings, Mo., Executive Director, Building Officials Conference of

America; Secretary and Liaison Officer, National Coordinating Council of the

Building Code Sponsoring Organizations; member. Joint Committee on Building

Codes . 63
engineering formulae, established standards, and proven practices.

Many of the standards are produced by trade associations in the

building industry, and some of these are written in code language

and presented to be copied in codes or incorporated by reference.

Federal agencies provide standards or requirements for their

own buildings or for buildings built by others with their

assistance. Similarly, state governments provide standards or regulations for their buildings and those for which they provide

assistance. State agencies also establish regulations for a variety of buildings for specific uses; and there is an increasing trend toward the establishment of state building regulations

which are effective state-wide.

Complete codes, referred to as "model codes," are published and

maintained by established organizations. These are adopted by

local governments, either by reference as published, or with

modifications to adapt them to local conditions allegedly

peculiar to the particular jurisdiction; or they may be used as the principal basis for locally prepared codes.

This basic difference also affects the systems for securing modifications

in the regulations. Changes in land-use requirements must be worked out with

each individual jurisdiction. For mitigation of loss from natural disasters,

this may be simplified by development of model sections for zoning laws and

recommended criteria for determining their appropriate application to local

condi ti ons . Changes in building regulations, especially those involving engineering

concepts, as in the case of regulations dealing with mitigation of loss from

natural disasters, may be approached through trade associations, Federal

agencies, state agencies or the organizations sponsoring the model codes.

However, success in effecting changes in these standards or codes does not

completely eliminate the need to promote comparable changes with individual

local governments and other authorities having responsibility for building

regu 1 ati ons

.

Investigation of conditions for this paper covered the following

specific matte rs :

. typical land-use control practices,

. philosophies and basic principles for development and

maintenance of model building code requirements,

. practices of local governments for adopting or applying the

model building codes,

. activities of other agencies and levels of government in

developing code requirements,

. the development of standards by a wide range of interests

and methods,

. local practices for plan examination, field inspection and

enforcement of land-use and building regulations, and

. possible impediments to effective enforcement.

Obviously all of these subjects are more general than the limited scope of

this workshop which is concerned with conditions dealing with natural disasters; but, except in certain subjects, we must consider the overall practices

since they apply to the particular conditions with which this paper deals.

64
Wherever it has been possible to confine the investigation to practices

relating to regulations for protection from natural disasters, as in the case of plan examination and field inspection practices, this has been noted

in the text.

Information regarding land-use control was obtained from the American

Society of Planning Officials, nationally known planning consultants, and

selected communities.

Information regarding the development of building regulations was obtained from the four code-sponsoring organizations: Building Officials

and Code Administrators International, International Conference of Building

Officials, Southern Building Code Congress, and the American Insurance

Association; and from the secretariat of the National Conference of States

on Building Codes and Standards. Information about standards was obtained

from published procedures of the respective organizations as applied to

codes activities as hereafter noted.

Information about local practices and possible impediments to effective

enforcement was obtained from published articles and by contacting local

jurisdictions in Missouri, Michigan, Massachusetts, Texas, and California.

The local jurisdictions contacted were selected to provide a range of experience from small medium, and large jurisdictions subject to various conditions

of exposure to natural disasters.

The Existing System

Any study of land-use and building regulations must be based on a clear

concept of their purpose, the legal authority and responsibility for their

adoption and enforcement, and the basis and manner of their formulation.

The primary purpose of such regulations is to safeguard the rights of

property owners, protect the life and limb of those who occupy or come in

contact with buildings, and secure the common good of the community by

minimizing the risk of disasters which would result in loss of physical

property, indirect economic hardship, or depriving the public of vital

services. It is now widely accepted that such regulations are necessary for

the public interest, but the distinction between public interest, consumer

interest, and individual interest is not always clearly drawn nor readily

recogni zed

.

The legal authority for these regulations is vested in state governments

[6]. However, because these laws so fundamentally affect the future of a community, the personal safety of its citizens and the property rights of

individuals, for many years it was considered desirable that they be enacted

and enforced by the unit of government closest to the people directly

affected by them, consistent with the ability of that unit of government to

establish efficient enforcement organizations. On this basis most states in

the past delegated much of their authority for land-use and building regulations to local governments, setting up the limitations on that authority in

the statutes governing the various types of units of local government [7].

However, the growth of metropolitan areas beyond municipal boundaries and

the increasing extent of production of industrialized construction marketed

nationally have led to an increasing trend to greater emphasis on the fundamental authority of the state governments.

(a ) Land-Use Regulations

Land-use regulations are a relatively modern concept of government

authority when compared to building regulations which have been in force for

many years. As we have already noted, the prevj,iling systems for developing

65
land-use regulations and building regulations differ. Since environmental

considerations must be the basis for land-use regulations, the prevailing

conditions have substantial effect on their nature in any community.

The influence of these prevailing conditions is reflected in the master

plan for the area. Standard principles for planning -- determining the most

desirable distribution, confinement, or prohibition of use of land within a municipality or area -- have been developed. To the extent these are followed

by planners, they may result in relativb uniformity of regulation of land-use.

Similarly, model sections for establishment of specific requirements are available. These are frequently used, in various combinations, to produce

zoning regulations for a municipality or area. As a consequence, there is a measure of uniformity in land-use regulations.

There is a growing trend toward greater flexibility in zoning requirements by use of special permits. This allows the governing body of a municipality to permit a broader range of use in a particular area or zone through

consideration of certain kinds of specific use on the basis of conditions

applying to the particular case. This practice may result in variations in

adjoining or neighboring communities.

The regulations of use of land to guard against loss from natural

disasters usually is covered in the master plan. Primarily it consists of

curtailing or prohibiting building on land judged to be susceptible to the

effects of natural disasters. Soil conditions, underlying geological conditions, danger of flooding, and similar conditions may be the basis for

prohibiting buildings in a specific area. Such areas may be devoted to park

or recreational use or left to provide open space.

(b ) Building Regulations

Building regulations in one form or another, date back many thousands

of years. They have been used for many different purposes, and have been

both praised and blamed for all kinds of conditions. Building codes are not

standards of good practice nor handbooks of construction. They are laws

prescribing the minimum that is acceptable for the safety of those who occupy

buildings and to minimize the hazard of one property to adjoining properties

[8]. Efforts to use them as guides for design or textbooks for construction

have resulted in confusion and misunderstanding.

Unlike land-use regulations, building codes are primarily technical

requirements. The background for code requirements is an extensive system

of design criteria, engineering formulae, and established standards combined

with records that have been kept through the years of building fires and

structural failures resulting from conditions of use and natural disasters

[9].

Standards are, therefore, one of the essential bases for building

regulations. But there is a fundamental distinction between "standards"

which are voluntary, and "codes" which are mandatory. If this distinction

is not carefully observed in the development of code requirements, confusion

may result and uniformity of regulations with the consequent protection

afforded the public, may be seriously impaired. This is especially important

in regulations for mitigation of loss from natural disasters.

For many years each local government had to rely on its own resources to

formulate the detail requirements of building codes. Lack of understanding

of true purpose for such codes and the nature of their restriction on individual property rights to accomplish community benefit resulted in their

being considered more of a necessary evil than important function of government. The resultant formulation of such laws by each community, more or less

66
hastily as the need arose to cope with some specific problem; and the

scattering of them through the municipal code, often under the jurisdiction

of various enforcement agencies, resulted in intolerable conditions.

In addition to these local regulations some states established codes

varying in scope from complete control over buildings, usually excepting

dwellings and farm structures, to regulations governing buildings for

particular uses. Like local activities, many of these were developed as the need arose, and their enforcement was spread among various agencies.

To cope with this situation, beginning in the early 1900's, various

organizations, agencies, and individuals tried a number of different methods

in a conscientious effort to establish a proper balance between the numerous interests, often conflicting, which must be served by building codes. These

methods included, among others, the development of code requirements

... by segments of industry which produced full codes or specialized portions of codes to cover their respective

interests ;

... by Federal Government agencies which attempted to produce

or sponsor development of standard building code requirements

for specific purposes or subjects;

... by State Governments which established regulations where local

governments had not done so, sometimes also applying in jurisdictions where codes were in force, resulting in overriding

or duplication of local authority;

... by organized industry-supported standards activities which

attempted to apply the principles used for developing

standards to produce standard building code requirements;

... and finally, by the model code system which provides

opportunity for open debate of controversial proposals

before volunteer committees which recommend approval,

modification or denial of specific wording as submitted.

Various approaches for full consideration of viewpoints to arrive at a consensus or decision on controversial provisions were tried in these activities. One of the basic problems encountered was that of securing public

agreement between the highly competitive segments of the industry. It was

a solution to this problem through the model codes system that resulted in

the progress which has been made in the development of codes.

Some of the former activities continue as an important part of this

system. Industry standards (some of which are referred to as codes or code

requirements) are developed by committees of researchers, professionals and

experts representing the industry and potential users. Strong efforts are made to achieve consensus among the industry and users.

Federal standards and state codes may be drafted by the staff of the

agency or by consultants. Drafts are published for review and comments by

all affected organizations, interests, or individuals. The resolutions if

conflicts may be accomplished through open forums, discussion with agency

staff, or by legal processes.

The Model Codes System

Under the model codes system four codes have been generally accepted.

These are frequently referred to as model codes in the sense that they are considered ideals as contrasted to the dictionary definition of "model" as 67
a "pattern to be copied, or for imitation or emulation," They are intended

for adoption by local governments by reference "without prejudice or local

amendment except as necessary to adapt the code to the administrative organization of the community" [10] or to justifiable local peculiarities.

Three of these model codes are sponsored by organizations representing

the officials who administer such codes in the various jurisdictions, with

provision for participation by the building industry. Originally these

codes were developed primarily to serve the needs of specific areas of the

country and to some extent they still reflect influence by typical conditions

of those respective regions. In those requirements affecting the use of

materials and engineering design there is a continuing trend toward standardization among these codes.

The fourth model code is sponsored by an organization representing a segment of the insurance industry.

While there is still some difference in the technical requirements of

these four model codes, one of the major differences is in the manner of

applying certain of the requirements relating to natural disasters. Of the

four nationally recognized model codes, two make earthquake requirements

applicable to all buildings, with certain exceptions, in variable degree

according to the recorded intensity of experience, in all communities where

the code is adopted. Another establishes "loss of life or damage of buildings resulting from earthquakes," [11] as shown by "local experience or the

records of the U.S. Coast and Geodetic Survey" [11] (which is now The

National Oceanographi c and Atmospheric Administration) as the basis for

application of earthquake requirements. The fourth model code does not

include earthquake requirements. To some extent similar differences exist

in requirements for wind loads. These requirements may be further modified

by local governments in adopting one of the model codes. The result is the

possibility of appreciable variation in the level of protection afforded the

public against the hazards from natural disasters.

To correct these conditions enforcement officials in state and local

governments, and Federal agencies must be provided with criteria for applying

disaster resistant requirements in their respective jurisdictions, and with

rational bases for use of these criteria in an optimum manner recognizing

the concept of risk and an uncertainty which is attached to the knowledge on any specific subject [12].

It is obvious, therefore, that there is some exigency for the establishment of uniform criteria for the application of regulations abating loss of

life, property, and vital services in all areas on the basis of applicable

contributing conditions. But regulations should only set the minimum requirements and must be sufficiently flexible to permit use of any materials or methods which have been proven to accomplish the prescribed level of protection. To maintain the proper relationship between code requirements and

standards there is need for a critical look at the substance of the codes

and the prevailing enthusiasm, especially in regard to engineering design,

for incorporating methods and formulae or limiting factors in the text of

codes. There is danger of this inhibiting the use of improved standards as they are developed.

The establishment of regulations, in itself, will not necessarily result

in the use of advanced technology. The design and construction professions

must be alerted to the advantages of advanced methods so that they will use them in the development of plans for buildings and structures. Design procedures and engineering formulae to aid and encourage the professions in the

use of the most up-to-date techniques are properly the function of the

professional and standards organizations. It is the responsibility of the

professional practitioners to make use of such standards to produce the best

possible solution to their client's problem, with full consideration to all

68
contributing conditions and within the scope of applicable requirements for

safety. The building official is responsible to check the professional's

designs to determine that they are within the scope of the regulations.

The procedures of the three building officials organizations for maintaining their respective codes up-to-date with progress in the building

industry are similar. The emphasis placed on these procedures by the several

organizations varies; and the impact of the updating process on local regulati ons also varies . Proposed changes come from a number of different sources: from local

officials, usually members of the organization; from committees of the

organization; from consultants usually representing some specific interests;

and from the highly competitive segments of the construction industry.

These procedures provide for submission of proposed changes in the

specific wording of provisions in the respective codes to the sponsoring

organizations. These are processed and published by the organization's

staff and are referred to committees established to study specific portions

or subjects of the code. The committee system varies and this difference

may have some effect on the final scope of the regulations.

After publication of proposed changes, such organization holds open

hearings presided over by committees, where proponents and opponents of each

change may present data and oral arguments in support of or opposition to the

proposed text and its ostensible effect on their respective interests. Based

on the information presented at the hearing and their collective judgment in

closed session, the committees make recommendations to the membership of the

organization for final disposition of the proposed changes. In two of the

organizations the final decisions are made by vote of the public official

members present in an open session of the organization's annual meeting; and

other submits the committee recommendations to its entire public official

membership for vote by letter ballot. After the membership has voted on the

proposed changes, those which are approved are published as supplements to

the codes. Complete new editions of the codes, incorporating all changes

approved subsequent to the previous edition, are published periodically. Two

of the organizations issue new editions every three years; and other issues

a new edition every five years [13].

The procedures of the fourth organization for determining code requirements and proposed changes are primarily staff functions. Meetings may be

called for discussion of proposals but final decisions are made by members

of the organization's staff. The staff is guided by the policies and principle interests of the membership of the organization.

The ultimate effectiveness of this system would be attained if every

local jurisdiction adopted one of these model codes "without prejudice or local amendment except as necessary to adapt the code to the administrative

organization of the community [14];" and updated the code as it applies in

its jurisdiction by prompt adoption of changes approved by the sponsoring

organization. Unfortunately, this is more the exception than the rule.

While these codes may be adopted as published by some smaller communities, they are seldom adopted without amendment of some sections other than

those related primarily to administration. In larger jurisdictions it is more likely that the model code will be used as a model for a completely

independent code prepared by a local committee or firm of consultants. In moderate sized jurisdictions, well-meaning members of local code committees

often persist in amending the model code as published so as to retain many

of the requirements of the old local code with which they are familiar. They

fail to realize that the model code was not written by an individual or group

of individuals in an ivy covered tower, but represents a sincere effort by

many organizations, agencies, and individuals to establish minimum regulations

69
necessary for safety and maintaining equitable consideration of the many,

sometimes conflicting and highly competitive interests involved [15].

It may be argued that these local amendments are necessary to adapt the

model code to local conditions, but too often they are influenced by the

personal experiences and individual prejudices of members of the local com- mittee, or result from pressure by local industries or other interests.

On the other hand, where climatic, geographic, geological, or other

natural conditions have a bearing on the application of standard or model

requirements -- such as frost line depth, valleys or natural corridors subject to peculiar wind velocities, earthquake prone areas, etc. -- local

amendments of model codes, or some other device such as an official rule or executive order, is necessary to fix the specific application of the model

requirements in the individual jurisdiction. Such amendments should be

limited, so far as practicable, to fixing the specific conditions to be

guarded against; such as minimum depth of footings for protection from frost

upheaval, minimum wind velocity or other load factor to be used in design

for protection against the hazard involved. Except for this difference, the

substance of the technical requirements in the model codes, which are based

on standards or engineering formulae, should not be changed.

Although it is contended that there is no substantial difference in the

technical requirements of the four model building codes, they all differ in

format, in the manner and extent of use of standards, and in certain primary

classifications which affect the application of many of the other code

requirements. Under certain combinations of conditions these differences

could result in appreciable variance in the protection provided the public

by the different model codes. These variances may be further aggravated by

the freedom of local governments to modify a model code in adopting it.

Code Enforcement Practices

In most jurisdictions land-use regulations and building codes are enforced by the local officer designated as building official. In examining

plans for use of land or proposed buildings the official must first determine

that the proposed use is in accord with the master plan of the community and

in conformance with the zoning regulations intended to implement that master

plan. If it is not, there is little reason for him to spend the time and

effort to examine the details of construction until the question of use has

been settled.

Few local departments charged with enforcement of such regulations are adequately staffed to fully discharge their responsibility effectively. In smaller jurisdictions individuals so engaged may be qualified primarily by

practical experience; while in the larger governmental units both education,

training and experience are usually prerequisites for employment.

Fortunately, the one time prevailing propensity for filling such

positions with political appointees as a reward for support, without serious

consideration of the appointee's qualifications, has largely disappeared.

The establishment of recommendations for organization of departments, job

descriptions, and personnel qualifications [16], has been a substantial

factor in accomplishing this.

Increasing emphasis on education of incumbent building officials, promoted by the professional organizations representing those officials, has

done much to improve enforcement practices and quality. But there is room

for additional improvement in this activity, especially in reference to

sometimes little understood requirements such as those referring to natural

disasters based on infrequent exposure.

70
There appears to be a range in the effectiveness of enforcement between

jurisdictions in highly populated areas and those where population is more scattered. This appears to be more the result of exigency than indifference

or inefficiency. As a result, some regulations may be enforced more energetically than others.

Such investigation of enforcement practices as was possible in the time

available in preparing this paper reveals that most officials check the basic

information used by the designer of a building -- such as bearing value of

soil, strength of framing elements, live and dead loads, wind load, etc. In the larger jurisdictions, where the staff usually includes professional

engineers, design procedures may be spot checked. In smaller jurisdictions,

the accuracy of design calculations may be often accepted on the integrity

and legal professional responsibility of the designing engineer or architect.

In jurisdictions where the code specifically applies earthquake load

requirements, the designs will be checked to determine that these requirements

have been observed. In jurisdictions where the code does not specifically

apply earthquake load requirements, especially in areas in a zero or number

one hazard zone, it appears to be a generally accepted philosophy that wind

load requirements provide sufficient protection against earthquake hazard.

In areas where there is hazard of greater intensity, failure to require

protection against possible damage may be serious.

In the investigation of the application and enforcement of earthquake

requirements the practice in several communities in southeastern Missouri

was checked. This area is rated as a number three hazard zone. In one community it was found that there were no building codes in force. In spite

of the fact that in the past two years this community has experienced noticeable quakes it is claimed there has been no serious damage to buildings. In the town proper there appeared to be no buildings over two stories in height

except the old stone courthouse. Outside the immediate town, however, a new industrial park complex is being developed in which there are several tall

buildings and stacks. No detailed information was available concerning the

design of these buildings. In two other communities checked in this area,

both having formal code enforcement, one does not apply earthquake requirements and the other does.

Field practices for inspection of buildings varies in almost direct

proportion to the plan examination practices. This appears to be strongly

related to the training and experience of the personnel. In the concentrated

portions of a large metropolitan area, field inspection is more comprehensive

than it is in small jurisdictions on the fringes of these areas or removed

therefrom. In one county investigated, which is not yet heavily populated

but is included in a large metropolitan area and in the direct line of growth

in that area, it was found that little attention is paid to the structural

design of buildings and field inspection is cursory, even though building

codes have been in force in the jurisdiction for well over five years.

The effectiveness of inspection and enforcement practices appears to be

influenced largely by internal conditions such as adequacy and quality of

staff. The overlapping of authority between agencies and the several levels

of government is more of an harassment to owners and designers than an impediment to effective enforcement. For example, an architect designing a nursing home must examine applicable requirements of Federal, state, county,

and local governments, both for their effect on construction and for health,

safety and sanitary conditions. He must determine the most restrictive of

these and design the facility to meet them. Often this may necessitate timeconsuming direct contact with a number of different agencies at each level of

government in an effort to resolve conflicting requirements.

Cumbersome procedures for prosecution of offenders -- crowded court

dockets, continued delays, suspended sentences -- often seriously impair

the effectiveness of enforcement of codes. Local influences may have some

71
adverse effect, especially where there is opposition to regulations or when

appeals to local boards or commissions are involved.

Enigmas in the Existing Codes System

No criticism of the present codes system in the United States can detract from the impressive progress that has been made under its influence.

It is built upon the doctrine of free enterprise which is the essence of the

American Economy. The evolution of the present codes methods is a fascinating

and inspiring study. It is hard to imagine the chaos that would exist, with

the modern materials and advanced techniques now available for building, if we had to depend on each local government developing its own regulations

instead of the development of model codes or other broad based regulation

systems through procedures which provide open forums for discussion of

opposing views.

In spite of this, building codes and related regulations continue to be

blamed for a wide variety of ills. It appears evident, therefore, that good

as the present system is, there must be some conditions or methods which

could be improved. One of the main problems in the past has been a certain measure of complacency with the present system. Those involved seem to feel

that they have a satisfactory working arrangement with each other and they

are skeptical of any efforts to change it.

Some of the other conditions under the present model codes system which

may be worthy of study are:

... It depends heavily on voluntary committees of men whose

principal responsibility is to the local communities they serve. While these men are dedicated to their respective

organizations' activity, much of their organization work

must be done on their own time and must be shared with

other responsibilities also having claims on that time.

... It does not use available industry and professional expertise

to the best advantage. Instead, it incites and encourages

the ruthlessly competitive nature of the several segments

of the industry to seek preferential consideration for their

respective vested interests.

... In the limited time available, the volunteer committeemen

can do little more than review the selected competitive

information presented to them by industry interests instead

of probing into the technical data on which sound regulations

should be based. The result too often is a compromise

between those competitive interests based on the scope of

personal experience of selected individuals comprising the

commi ttees

.

... It lacks specific policies fixing the proper scope for codes,

definition of terms relating to code work, and establishment

of a proper relationship between standards and codes. The

incorporation of design data and engineering formulae into

the codes as requirements, instead of confining regulations

to performance requirements and acceptance criteria, leads

to variances which may significantly affect the protection

provided to the public.

... It lacks meaningful communication with related governmental

agencies to encourage acceptance of the model regulations

and prompt use of available improvements in code requirements.

72
Many of the individuals who serve on committees or vote on code requirements cannot or do not follow the organizations'

recommendations in their own jurisdictions.

... It lacks clear definition of the respective proper roles for

the various levels of government -- federal, state, county,

local -- resulting in overlapping of jurisdiction and superimposing of regulations.

The code change cycle varies among the several organizations from nine

to twelve months from final date for submission of proposed changes to date

of official action. Time for publication of supplements varies from two to

four months. Since the time for submission of proposed changes varies from

January to August, and the time for final action varies from September of one year to June of the following year, it could require approximately

eighteen months to process a specific change through all of the model codes

change cycles, assuming that the proposed change is processed within the

cycle in which it is submitted. Complicated or controversial changes may

be held over from one code change cycle to another by any one of the organizations, thereby substantially increasing the time for total acceptance.

In 1971 one of the model code organizations modified its procedures to

place consideration of proposed changes in its codes in alternate years, and

banning consideration of changes every fifth year when new editions of its

codes are scheduled. Under this schedule a lapse of three years may occur

in consideration of changes for that code.

There are other conditions in the present codes system which might be

improved with study so that it would better serve the public interest without

bias or prejudice. The existence of these conditions in the present system

is not an indication of indifference, but rather is the result of the heavy

demand on the participating agencies to cope with the accelerated advancement

of new technology, although the situation is aggravated by jealousies between

the various interests involved. There is need for the establishment of an unbiased coordination of the existing efforts, in a manner commanding the

respect of all concerned, to use the vast resources and expertise available

in the best possible manner.

The nature of the governmental structure in the country, the basic

principles of the economic system, and the wide variety of conditions and

interests which are affected by land-use and building regulations are not

conducive to voluntary cooperation. The resultant rivalry between the

several interests, between levels of government, and among government agencies

has long been one of the major problems in code activities. There has not yet

been established a meaningful effort toward coordination of the activities and

programs of the many interests which presently are endeavoring to achieve

unity, each in its own way. Fortunately, very recent activities, discussed

later in this paper, show recognition of this and offer prospects for some improvement

.

Problems in Effecting Improvements

As has already been pointed out, improvement of building regulations

relating to mitigation of loss from natural disasters will not, in itself,

necessarily result in the use of advanced technology. However, the regulations must be such that improved technology which has been proven to

accomplish the prescribed level of protection, may be used.

The first step in effecting changes in code requirements covering

natural disasters, if needed, is the development of technical data from

which the basic principles for code provisions can be established. From

these basic principles, performance criteria fixing the acceptable levels

73

494-610 0 - 73 -6
of safety should be developed. In the case of safety from the hazards of

natural disasters the application of requirements may vary from area to area depending upon the magnitude of the hazard in each area. Coincident with this code activity, and perhaps preceding it if the

basic principles involve engineering design criteria or formulae, the

standards organizations responsible for development of standards must be

contacted and the necessary criteria and formulae established. Promotion of

the use of these standards may be accomplished through the organizations

representing the design professions.

With performance criteria established, acceptable levels or risk fixed,

and engineering design criteria or formulae developed, performance requirements can be prepared for protection against the specific hazards. The

development of these and of modifications in state or local codes, or in the

model codes, is governed by the established procedures of the government,

agency or code-sponsoring organization. If the requirements affect any of

the competitive segments of the industry, opposition or counter proposals

may be expected.

Changes in a state code may become effective immediately upon adoption

or at a date fixed in their adoption. After changes have been approved by a model code organization, there remains the need for securing acceptance of

them by local jurisdictions which are operating under that code. While the

sponsoring organization promotes the use of approved changes, the most

effective method for securing meaningful use of them is by direct contact

with local governments. There are also those individual jurisdictions which

have not specifically adopted or used one of the model codes. Each of these

must be contacted, usually with specific changes in their individual code.

Present Efforts and Recommendations

Since many of the conditions contributing to the hazards from natural

disasters extend beyond the arbitrary boundaries of local jurisdictions,

there is need for voluntary cooperative action or control by area-wide

regulation. This is also desirable where the development or expansion of

one community may affect natural conditions so as to cause flooding or in some other manner be detrimental to other communities in the area or communities some miles distant. Conditions extending beyond the borders of one community into the area of an adjoining community should be similarly con- trolled in both jurisdictions.

It is unrealistic to expect this to be accomplished by unilateral action

of the affected communities. Some states have recognized this and have taken

steps to set up land-use guideline regulations. In other areas metropolitan

planning agencies have been established to coordinate land-use requirements

in all of the communities in the area. The same principle has not been widely

applied to building regulations.

Recognizing this, and the need for encouraging the use of new materials

and advanced technology in building construction, the National Bureau of

Standards some years ago assisted in the establishment of the National

Conference of States on Building Codes and Standards. This was done in

response to requests from several state governments which began to recognize

their basic responsibility for such activities.

In the reports of the committees of this organization, and official

actions and resolutions approved at its recent meeting [17] the National

Conference of States on Building Codes and Standards proposed activities

intended to increase unity in building regulations, pledged its cooperation

to this end, and called on others to help. Some of these proposals were:

74
... greater cooperation among Federal agencies developing,

promulgating or enforcing rules and regulations affecting

building construction so as to eliminate the present

rivalry, duplication of effort and superposing of regulations ;

... greater cooperation among state agencies having authority

and responsibility for regulation of buildings so that

there may be meaningful reciprocity between the states in

the acceptance of products and systems manufactured and

marketed on a national basis;

... greater unity in building regulations by the adoption of

one of the four nationally recognized model building codes,

without change, by state agencies which have the authority

to adopt mandatory or optional state building codes.

In support of these resolutions the National Conference of States on Building

Codes and Standards has activated committees on

... Standards and Evaluation, dealing with standardization of

testing, evaluating, and continued quality control of

building products;

... Education and Qualifications, dealing with building

regulation enforcement personnel;

... Management and Regulatory Procedures, dealing with the

organization for building code enforcement; and

... Reciprocity, dealing with the mutual acceptance of both

enforcement personnel and approved products among the

states

.

These activities recognize the responsibility of state governments for

regulations providing for safety to the public in the use of land and the

construction, use and maintenance of buildings. They acknowledge the value

of the model codes system and the advancement toward uniformity which has

been achieved through it. They are directed to bringing about unity in the

preparation, maintenance, and enforcement of land-use and building regulations,

administrative techniques and personnel qualifications at the local level

through the exercise of state authority.

The present emphasis on the authority of local governments for building

regulations was based on conditions when it seemed that home rule could better

serve the people through the cities because they were widely separated and

the economy was largely local. But the country has outgrown these conditions

and the bulk of the population is now concentrated in metropolitan areas.

This change necessitates readjustment in the codes activities of the country.

The model code organizations anticipated this many years ago by forming

regional associations which produced model building codes suited primarily to

the needs of the regions they served [19]. These are the major elements of

the present model codes system.

These organizations have recognized the need for closer coordination of

their respective activities to cope with the changing conditions in the

country. In the past few years they have voluntarily increased cooperative

efforts among themselves. Through these efforts they have jointly:

... produced a model One and Two Family Dwelling Code,

developed a proposed model Residential Rehabilitation

Standard,
begun work on a similar standard for Mobile Homes,

... reorganized the Joint Committee on Building Codes and

strengthened its activity through the Model Codes

Standardization Council,

... cooperated in the development of model standards for

evaluating industrialized building systems,

... and, most recently, have revived former cooperative

efforts by forming a Council of American Building

Officials to provide a unified voice on matters of

national importance.

Under the Model Codes Standardization Council work has progressed on proposed standardization of [18]

... The Format for Building Codes

... Definitions Used in Building Codes

... Classification of Occupancy

... Types of Construction

Within the past few months there has been a major breakthrough in

cooperation by a number of groups. This involved the consolidation of

independent efforts by the Model Code Groups, the National Conference of

States on Building Codes and Standards, the National Association of Building

Manufacturers, the Department of Housing and Urban Development, and the

Council of State Governments to produce a Model Manufactured Building Act.

This has been published by the NCSBCS, and when adopted by state agencies

having authority and responsibility to implement state standards, will

result in acceptance of manufactured buildings on a nationwide basis regardless of where they are manufactured [20].

The activities of the National Conference of States on Building Codes

and Standards offers promise of progress in the standardization of code

requirements. If the state authorities make use of the model building codes

as recommended, considerable progress can be made in a short time. But the

state activity does not fully cope with the problem of large metropolitan

areas which overlap state boundary lines. There is a growing recognition of

the importance of involving Regional Councils of Government in land-use

planning and coordination of building regulations [21].

In addition to the activities of local and state governments which have

been described, a number of Federal Government agencies have rules, regulations, codes, or other activities which affect the design and construction

of buildings. Some of these are:

Department of Housing and Urban Development -- FHA

Minimum Property Requirements

Operation Breakthrough Guide Criteria

Department of Health, Education and Welfare

Health Care Facilities

Educational Facilities

General Services Administration

Federal Specifications

Guidelines and Requirements for Federal Buildings

76
Department of Labor

Occupational and Health Safety Act

These Federal activites generally focus on standards for buildings

constructed by Federal agencies for their own use or constructed for other

use with Federal assistance. Controversies have arisen concerning these

requirements in areas where they are at variance with local codes, which may

be more or less restrictive than the Federal standards. Although in some cases the application of local regulations has been insisted upon by local

authorities, the threat to withhold Federal support for the building activity

has been effective in encouraging use of the Federal standards.

These controversies have aroused concern that Federal standards will be

imposed upon local codes covering building that is not Federally assisted,

and there have been efforts by some Federal agencies to impose such standards,

indeed to impose language which is not documented by widely accepted standards.

This activity is of extremely dubious constitutionality.

The cumbersome, time consuming necessity of having to prepare specific

code requirements for several model codes; promote their adoption by several

organizations, and examine their application in local jurisdictions at the

state level and in the rules and regulations of numerous Federal agencies;

and the need for the repeated pitting of one interest against the other, is

wasteful of valuable expertise and human resources. It results in substantial

delay in effecting modernization of regulations and providing the public with

adequate protection.

It is obvious, therefore, that the present model code system can and

should be improved. Experience has shown that any effort to accomplish this

must be undertaken so far as practicable within the framework of the system.

It can be done without drastically affecting the several organizations or interests involved in the present system, although their respective roles and

particular activities may have to be adjusted. The principal aim should be

a coordination of those activities which now are a duplication of efforts,

providing opportunity for greater emphasis on other activities.

The shift of basic effort from preparation of precise code requirements

by competitive industry interests for four model codes, local codes and state

and Federal agencies, to the development of universally accepted basic

principles for such regulations will no doubt reveal a need for specific

basic research for the development of standards. Some of these standards may

differ from the type of standard with which we are accustomed to dealing.

Material standards, engineering standards, test standards, and safe practice

standards are well known. These have been developed under well established

standards procedures maintained by recognized organizations. But in addition

to these standards, the development of basic principles for code requirements

requi res

... standards of safety -- minimum levels of safety, in clearly

defined terms, desirable in buildings to protect the public

against the hazards involved;

... evaluation criteria -- the basis for determining the performance of a material, product, or assembly by analysis

according to accepted engineering design formulae or by

test; and

... performance criteria -- the basis for establishing desirable

levels of performance for materials, products, systems,

assemblies, or elements of buildings.

The need for standards of safety is especially important in establishing

the basic principles for code requirements dealing with natural disasters.

77
The magnitude of hazard differs from area to area in proportion to measurable

severity of exposure. This necessitates the gathering of pertinent data;

assembling it in usable form; and establishing definitions for words and

terms to be used so as to facilitate communication in regard thereto.

The first step in applying these principles of improvement to the

standards and model codes systems should be the establishment of a council

independent of the various levels of government and the organizations cur- rently involved in the state and model code system; but having their full

respect, confidence, cooperation and participation. This council should

sponsor the coordination of all codes activity affecting the construction of

buildings and their maintenance and use. It should provide liaison with

research and standards activities, and with Federal, state and local government representatives.

This council should be made up of individuals from all interests con- cerned with building codes and the development and use of new products or construction systems or techniques; selected from the several participating

interests on the basis of their qualifications and apparent ability to

perform constructively and without bias in the activities to the council.

The purposes of the council should be to pull together existing data,

and sponsor development of new data where necessary --

... To recommend a national policy concerning the building codes

system clearly stating: The purpose of codes; the codes to

be considered; the proper role for various levels of government; the responsibility of governmental agencies; the

desirable administrative organization and personnel qualifications for code enforcement; and the basis for determining

code requirements.

... To develop a positive program for implementing that policy,

providing for: review of the existing codes activities and

problems therewith to determine how they may be coordinated

and improved; standardization of administrative techniques

and personnel; prompt dissemination of information to all

concerned; and public relations and public education

acti vi ties .

In the development and implementation of this program the substantial

accomplishments by the code-sponsoring organizations, and the responsibilities of the several levels of government must be recognized. The object

must be to enhance these, coordinating them to minimize duplication of

effort, and expanding them where necessary to make use of the resources of

manpower and expertise to the best possible advantage.

Summary

It is concluded from this study that there is need for greater uniformity

in the regulations presently in force for protection against loss from natural

disasters. There is special need for more precise criteria for applying such

regulations in individual jurisdictions; particularly with reference to the

hazards from earthquakes. To accomplish this, a rational system or formula

is needed for establishing a balance of risk since absolute protection may be

impracti cabl e

.

The evolution of the development of land-use and building regulations in

the United States has produced a codes system that is responsive to national

needs. Although the codes activity has been blamed for all sorts of ills

besetting the construction industry and the public, careful examination of

these has shown that they are more the result of isolated incidents than

78
gaping faults in the present system. The organizations which have been

active in the development of this system are deserving of admiration and

must be given substantial credit for their accomplishments.

The envolvement of the elements of this system on a regional basis, has

resulted in certain conditions that require considerable duplication of

effort. This has also resulted in some variations which make compliance

with the codes cumbersome and sometimes unnecessarily costly under the present shift of emphasis to national production of materials and products,

including major components of, or complete buildings.

There has been a growing awareness of this by all concerned and in

response to it the model code organizations have voluntarily increased

cooperative efforts among themselves. They have jointly issued a model code

for residential construction; they have collaborated on the development of a proposed residential rehabilitation standard; they are working on a cooperative effort for evaluating industrialized building systems. For years they

have worked together in the Joint Committee on Building Codes, now the Model

Codes Standardization Council, comparing, studying, and attempting to resolve

some of the fundamental differences between their codes. But these activities

have been established as individual elements of the program as the need for

them has arisen, rather than as part of a well planned, long range program

for improvement of the present codes system. In effect, they duplicate one

of the major problems, which is compounding of effort.

State authorities also have recognized the need for greater responsibility on their part and have formed the National Conference of States on Building Codes and Standards. This organization has made an effort to

develop a meaningful working relationship with the sponsors of the model

codes and with those agencies or organizations which develop standards.

In recent months these cooperative efforts have resulted in a substantial breakthrough producing a Model Manufactured Building Act. This

development should result in the acceptance of sound new building techniques

on a nationwide basis.

Although there was Federal agency participation in this, there appears

to be less effort at the Federal level to coordinate the activities of the

many agencies involved. Perhaps one Federal Government agency should be

designated as the control agency for all Federal Government rules, regulations, codes, standards, etc. To a large extent this has been done with

Federal Specifications, why can it not be done with codes and standards?

Within the legislative branch of the Federal activity there is also

need for much improvement. A Legislative Clearinghouse through which all

bills may be checked to eliminate multiplicity, may be the answer. Above all, there is need for an unbiased, critical examination of the

present codes system to determine what improvements can be made in it and

how these can be best accomplished without discrediting or destroying the

impressive progress which has already been made. The need for this is

significantly evident in requirements regarding mitigation of loss from

natural disasters over which we have no finite control. Such an activity,

to be successful, will require overcoming the complacency with the present

system, and the foregoing of individual interests in a truly cooperative

spirit to produce results that will genuinely serve the public interest and

provide the safety to which all are entitled without imposing unnecessary

restraint or unnecessary expense.

79
References

1. Description of program re: Building Practices for Disaster Mitigation,

National Bureau of Standards, May 2, 1972.

2. BASIC BUILDING CODE as published and maintained by the Building

Officials & Code Administrators International, Chicago, Illinois.

3. SOUTHERN STANDARD BUILDING CODE as published and maintained by the

Southern Building Code Congress, Birmingham, Alabama.

4. UNIFORM BUILDING CODE as published and maintained by the International

Conference of Building Officials, Whittier, California.

5. NATIONAL BUILDING CODE as published and maintained by the American

Insurance Association, New York, New York.

6. Chalres S. Rhyne, Survey of the Law of Building Codes, (Washington:

The American Institute of Architects and National Association of Home

Bui 1 ders , 1 960 ) , pp . 6 & 7.

7. Rhyne, Survey of the Law of Building Codes, (Washington: 1960), p. 7.

8. Paul E. Baseler, "Basic Principles of Building Codes," Local Building

Regulations, ed. Paul E. Baseler, (Chicago: Building Officials

Conference of America, Inc., 1963).

9. Richard L. Sanderson, "Codes and Code Administration" (Chicago:

Building Officials Conference of America, Inc., 1969).

10. "Seven Point Program", National Coordinating Council of BOCA, ICBO,

and SBCC, (Washington, D.C. 1964).

11. Sec. 719.0 Earthquake Load, Basic Building Code, (Chicago: Building

Officials and Code Administrators International, 1970).

12. J. H. Wiggins, Jr. and D. F. Moran, Earthquake Safety in the City

of Long Beach Based on the Concept of Balanced Risk, (J. H. Wiggins

Company , Pal OS Verdes Estates, California, 1971).

13. Procedures and By Laws of the code-sponsoring organizations, (Chicago:

Building Officials & Code Administrators International, Inc., 1971;

Pasadena: International Conference of Building Officials, 1971;

Birmingham: Southern Building Code Congress, 1971).

14. "Seven Point Program", National Coordinating Council of BOCA, IBCO,

and SBCC, (Washington: 1964).

15. Richard L. Sanderson, Is 'Pride of Authorship' a Threat to Adopting

Modern Building Code? (The Building Official , January 1968, Chicago:

Building Officials & Code Administrators International, Inc.).

16. Building Department Recommendations: (Chicago: Building Officials

and Code Administrators International, Inc., 1968).

17. News Release and attached record of 5th Annual Conference of the

National Conference of States on Building Codes and Standards,

(Washington: National Conference of States on Building Codes and

Standards , May 1 972 ) .

18. Appendix A to Report of Standards and Evaluation Committee of the

National Conference of States on Building Codes and Standards to 5th

Annual Conference, (Washington, 1972). , 80
19. Eugene B. Piester, "A Cooperative Effort", Building Standards, May-June

1972, (Whittier, California: International Conference of Building

Officials)

.

20. Piester, Building Standards, May-June 1972.

21. Newsletter, National Association of Regional Councils, July-August 1972,

(Washington: National Association of Regional Councils).

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards, August 2 8 -S eptemb er 1,

1972, Boulder, Colorado (Issued February 1973).

81
EARTHQUAKE HAZARDS FOR BUILDINGS

by

Neville C. Donovan*

I. Introduction

When the ground is shaken by an earthquake, a building resting on the

ground must respond to the adjacent ground motion. Damage produced by earthquakes has provided some insight into how buildings respond. This information

has been greatly augmented in recent years by theoretical and experimental

studies of structural response to random vibratory loadings similar to those

produced by earthquake motions.

The ground motion to which the building responds can be considered from

two different aspects. These are, the general geographic region in which the

building exists, and the close-in environment where the building can be

influenced by the local conditions. This review of the earthquake loading

on buildings considers the effects of these two different aspects and how

they are related. The areas where significant knowledge exists are described

and illustrated by examples of their use in design practice. Areas where

information is deficient are also outlined. Some areas where a research

effort could efficiently provide useful information are described.

1 1 . Regi onal Effects

Seismic Environments

Consideration of the regional aspects of earthquakes includes the

disciplines of geophysics, seismology, geology, and engineering. The more frequent occurrence, of earthquakes in some areas than others is readily

apparent from any review of sei smol ogi cal data. The instrumental epicenters

of events with Richter magnitudes greater than about 4 between 1961 and 1967 were compiled and plotted by Barazangi and Dorman (1969). The data have since

been extended through 1969, and a map has been published by the Environmental

Science Services Administration (N.O.A.A.) of the Department of Commerce.

This map has been reproduced in Figure 1.

The emerging field of plate tectonics has provided considerable insight

to aid the understanding of the major global sources of earthquakes. Plate

tectonics and the associated relationships of sea floor spreading and con- tinental drift are beyond the scope of this review. A reference by Bullard

(1969) is suggested as a general introduction for the interested reader.

Concentration of epicenters around some of the major continental blocks and

along the mid-ocean ridges can be readily seen in Figure 1. The large con- centration of events on the west coast of the contiguous states and the

southern coast of Alaska can be seen in Figure 1. The heavy concentration

of events in the area south of Anchorage, Alaska, includes the main event and

aftershock sequence of the March 27, 1964 Prince William Sound earthquake.

A linear relationship between large and small magnitude earthquakes, when

the number of earthquakes are plotted logarithmically, was demonstrated by

* Partner, Senior Engineer, Dames & Moore, San Francisco

82
83
Gutenberg and Richter (1949). A recent compilation by Evernden (1970) has

shown that the relationship is valid for seismic data obtained from all

seismic regions. A representative plot of this type is shown in Figure 2 for data obtained over a 163-year period within 100 kilometers of the downtown San Francisco area. Where historical data are available, records of

small earthquakes can be of considerable value in estimating the level of the

seismological hazard.

The risk of exposure to earthquakes is greater in regions where earthquakes occur most frequently. This simple premise is complicated by two

separate factors. The first of these factors is the known occurrence of

catastrophic earthquakes in other portions of the United States, such as the

1811-1812 New Madrid earthquakes in Missouri and the 1886 Charleston, South

Carolina earthquake, where the overall seismicity is lower.

The second factor is more complex. Unlike floods or wind storms which

either take time to develop or give indications of their approach, earthquakes

occur without warning. The lack of warning makes a severe earthquake a more frightening event than it might otherwise be. The relative infrequency of

major earthquakes and their occurrence without warning when combined with the

lack of knowledge of the mechanisms which cause them have delayed the approach

to seismic design on the same logical basis using the probable risk of exposure

as is applied to other variable phenomena such as floods.

The concept of dual earthquake levels for design has evolved over the

last decade. This was developed initially for nuclear power plant design

requirements. The upper level represents a large earthquake with a low probability of occurrence. Should this earthquake occur, non-critical parts of

the plant may be damaged, but the plant would be able to close down without

any potential radiation hazard. The lower level represents a smaller earthquake with a higher probability of occurrence. The plant would be expected

to remain operational during and following this smaller earthquake (Newmark

and Hal 1 , 1969 ) .

The seismic design criteria for nuclear plants are much more stringent

than those applied to other structures, and their direct use for other

structures is not recommended. Nuclear power plants, where consequences of

an accident are greater, are designed much more conservatively than buildings.

This difference can be illustrated by considering the roughly estimated levels

of probability of failure in a modern building. This probability is of the

order of 10"° (Ang and Ellingwood, 1971). For nuclear power plants, the

unpublished probability has been estimated to be lO'S, one thousand times

less than the number for buildings.

The dual earthquake concept when applied to buildings represents a more complex interaction of probability, design conditions, and risk levels. However, there are within the present state-of-the-art differences of opinion relating to the choice of the level of earthquake motion for design. For this

review paper, the two design level earthquakes are simply designated. Level I and Level II.

The Level I earthquake represents an event with a low probability of occurrence or a long return period. Such an event has only a low probability

of occurring during the life of the proposed structure. Should such an event

occur, it is anticipated that the building would respond with excursions of

the structural frame into the yield range. The structural behavior should be

such as to minimize the potential loss of life from collapse or failure of

non-structural elements. Damage to the building could be extensive enough to

require major repairs.

The Level II earthquake represents an event which has a moderate probability of occurring during the lifetime of the structure. The response of

the structure to the Level II motion should result in only minimal damage to

84
non-structural elements. Although not required, it is prudent to assume that

the structure will respond within the elastic range. The economic relationship between the possible costs of minimizing non-structural damage and the

possible costs of repairs of this damage and the effects upon the public

welfare should be balanced for the Level II earthquake.

Procedures have been developed where the probable level of exposure to

earthquakes can be evaluated from the seismic history of the region. Interpretation of the seismic history must be made with recognition of the asso- ciated geologic and tectonic information. The computed exposure can be

expressed in terms of probable levels of seismic motion which are in turn

used to estimate response levels of a building and the possible consequences

of the response. Comparative consideration of the costs of repairing possible

damage, and costs of preventive design, and the effects upon the public com- munity, allow the risk analysis to include an economic evaluation.

In this paper, a suggested procedure is described in some detail with

alternative methods described where possible. It is recognized that the full

procedure described cannot be economically applied to the design of smaller

buildings. Some suggested improvements and modification to the lateral force

design code are suggested. It should be stressed that the use of present

codes as the sole basis for design of a major building is not in accordance

with the current-state-of-the-art. However, observance of a design code is

usually a minimum standard required by regulatory agencies.

The science of instrumental seismology dates from the late 19th Century.

The first recording seismographs in the United States were installed at the

University of California and the observatory on Ht. Hamilton in 1887. The

history of large earthquakes is much longer. Some Chinese records have been

compiled representing 3,000 years. In the United States, historical records

range from about 70 years for Alaska to 400 years for areas along the Atlantic

coast. Early seismic histories often must be compiled from old newspapers,

books and other records. The early records are quite subjective in nature

but provide valuable extensions of more recent instrumental records of both

large and small events. Compilations of old records have been made and

published for some portions of the United States. An excellent example of

such a compilation is the documentation of earthquakes on the Pacific Coast

of the United States between 1769 and 1928 by Townley and Allen (1939).

When combined with detailed geologic data, the probable earthquake

source mechanisms can often be identified. Where not identifiable, it is

often adequate to assume that the source may be randomly located within the

re g i 0 n .

Risk Evaluation

Seismic risk should be expressed in terms of return periods such as is

done for winds and floods. The basic sei smol ogi cal information obtained from

the seismic geology and the available seismic record should be converted into

terms suitable for use by the engineer. Magnitude information or even Modified Mercalli intensity values can be expressed as return periods or probability curves rather than as ill-defined terms such as "probable maximum"

or "lifetime earthquake." For a well-balanced economic design it is necessary

to know how quickly the risk of occurrence decreases as the intensity of

ground motion increases. It has been found empirically by several investigators (Nordquist, 1945; Milne and Davenport, 1965; Dick, 1965) that the

distribution of maximum magnitude or intensity in a region can be represented

by an extreme value distribution of the Gumbel (1958) type. Cornell (1968)

showed that the double exponential distribution, which is widely used in

engineering studies of extreme events, could be derived from accepted seismo1 ogi cal relationships.

85
1 000 r 1 1 \ 1 r

UJ

2 3 4 5 6 7 8 9 RICHTFR MAGNITUDE

FIGURE 2 REGIONAL SEISMICITY

163 YEAR RECORD

{MAGNITUDES GREATER THAN 4.0

WITHIN 100 KILOMETERS OF SAN FRANCISCO)

86
The simple extreme relationship was extended to a continuous range of

intensities of motion and return periods over regional areas (Milne and

Davenport, 1965, 1969; Lacer, 1965). Correlation with the known seismic

geology was difficult with these procedures. A somewhat different approach

giving direct relationships between various assumed sources of earthquakes

was produced by Cornell (1968). This procedure has since been extended to

consider restrictions such as finite limiting magnitudes on different sources

(Cornell & Vanmarcke, 1969; Cornell, 1971). The Cornell procedure and the

procedure developed by Milne and Davenport for the seismic zoning of Canada

have both been used by the writer on the San Francisco example. Where the

Cornell procedure is restricted to assumed areal sources only, and the same acceleration attentuation law is used in both procedures, the results are comparable. In addition to giving similar results to the method developed

by Milne and Davenport in regional cases, the Cornell procedure is distinctly

superior in areas where the fault locations are well defined.

The San Francisco epicentral data were plotted as shown in Figure 3. For a seismic risk evaluation, the events are more easily considered as originating from distinct sources. A simple four source model was chosen.

The four sources were three fault zones, the San Andreas, Hayward and

Calaveras, and an areal source to the north where the fault locations are

less distinct. Details of the source modelling away from the immediate

vicinity of the site of interest do not greatly affect the risk estimates.

The events located within the bounds shown on Figure 3 were assigned to the sources shown. It was also assumed that the maximum magnitude of possible

events from all sources except the San Andreas Fault would not exceed 7.0.

On the San Andreas Fault, events with magnitudes as large as 8.5 might occur.

Cornell and Vanmarcke (1969) showed that the risk estimates were very

sensitive to the slope b in the Gutenberg and Richter equation (log n = a-bm;

where n is number of recorded events and m is the magnitude) and to the

attenuation equation values. Evernden (1970) assembled a worldwide set of

data relating magnitude and numbers of events. Where data are complete, the

value of the coefficient b usually has a value between 0.65 and 1.4. The

data plotted in Figure 2 have a b coefficient of only 0.53. It is believed

that the smaller value for b in this example is caused by the incomplete

recording of the smaller events in the almost 100-year portion of the record

prior to instrumental seismology. As the historical data were used to assign

numbers of events to each source mechanism, the coefficient computed from the same record was used in the numerical procedures.

Motion Attenuation

Attenuation of acceleration has been studies in several different ways.

Cloud (1963) and Cloud and Perez (1971) have suggested an envelope relationship between distance from the zone of energy release and the maximum

acceleration independent of the magnitude of earthquake. Other studies

have included the magnitude of earthquake as a parameter (Gutenberg and

Richter, 1959; Esteva and Rosenblueth, 1963; Blume, 1965; Kanai , 1965; Milne

and Davenport, 1970; Esteva 1970; Schnabel and Seed, 1972).

The 1971 San Fernando earthquake provided a valuable contribution to

earthquake engineering, as more than 100 strong-motion instruments recorded

accelerations from this one event. The large data set from the one event

allowed a study of attenuation free from the additional effects of source mechanism, event magnitude and regional tectonic variations. The author has

also accumulated strong-motion acceleration data from the United States,

Japan and world earthquake lists into a data set of more than 500 instrumental

acceleration values. These data are shown in Figure 4. Curves in Figure 4 represent the least squares fit between acceleration and distance where the

distance is modified by a constant value of 25 kilometers (Esteva, 1970).

The data were sorted and ranked after the method of Gumbel (1958) to obtain

87
39°

N

FIGURE 3

EPICENTER MAP

(HISTORICAL INSTRUMENTAL OR ESTIMATED EPICENTRAL

LOCATIONS WITHIN 100 KILOMETERS OF SAN FRANCISCO)

88
i

\

\

B MEAN t1 STANOARd\ *^ DEVIATION \

\ 1\ , \

6 Ea B B 5>;^ 5

B BB #

\^ 1 B- \ At ~

IT =

H 6B 6 6 \

CLOUD-PEREZ

ENVELOPE

nignitudl Izil bhin b . D nign,budi 5.0 bo E . 0 nig.n.budc B . 0 bo 10

nagnibudc 1.0 bo 8.0

nagn.fcudK graati- ch»n B . 0

HYPOCENTRAL DISTANCE IN KMS. FIGURE 4

PEAK GROUND ACCELERATION

IN TERMS OF DISTANCE FROM CAUSATIVE FAULT

WORLDWIDE SET OF 515 STRONG MOTION RECORDS

WITHOUT NORMALIZATION OF MAGNITUDE

89

494-610 O - 73 - 7
n

/' /

/

• / .

" /

• /

7^7^

//

//

//

/A rf

//

//

/ f

r

A

/

/

:' /

. / /

/ STANDARD OEV

/

lATlON = .92

3 . DO DD

I n m z z u I- e d c m p u b e d

Pull Jvks Bvfc MibhDub maqnibuds normaliufcion

. ggBg

gg

gg

gg

go

BD

IDBDBD

HD

gD

2D

ID

DB

D?

Dl

DDI

DDDI

DDUDI

3 . DD

FIGURE 5

DISTRIBUTION OF

MEASURED /COMPUTED PEAK GROUND ACCELERATIONS

WORLDWIDE SET OF 515 STRONG MOTION RECORDS

WITHOUT NORMALIZATION OF MAGNITUDE

90
the standard deviation of the data set. Dashed lines represent the one and

two standard deviation limits about the mean curve. As the data shown in

Figure 4 have not been normalized in any way with respect to magnitude, the

envelope curve developed by Cloud and Perez (1971) is shown as a lightly

dotted line. The standard deviation of this data set is 0.92. Normalization

of the data with respect to magnitude in the form first suggested by Esteva

and Rosenblueth (1963) slightly reduces the standard deviation of the data

to 0.84. A plot of the ranked data in Figure 5 to normal probability scaling

shows good linearity suggesting that a normal distribution is a good representation of the variation of the data. The resulting attenuation equation

for surface acceleration maxima obtained by the author using least squares

procedures for the three variables is:

y = 1 320 eO-^S'" (R + 25)"^

2 where y = the peak acceleration in cm/sec

m = the Richter magnitude

R = the hypocentral distance, the distance

to the causative fault or the distance

to the center of energy release, if

known, in kilometers.

The small reduction in the standard deviation when the magnitude was normalized in the data set is not considered as an indication that acceleration attentuation and magnitude are not related. While it is believed by

many writers (Housner, 1965; Bolt, 1972) that maximum acceleration values

close to the causative fault are independent of magnitude, the attenuation

away from the fault is dependent on magnitude and distance. The assumption

that magnitude and distance can be considered as independent variables is

believed to be the reason for the small reduction in the standard deviation.

Theoretically, the exponential coefficient relating acceleration values and

distance, without consideration of material damping and dispersion, should

vary between 2 for small magnitude earthquakes and 1 for very large magnitude

earthquakes. The dependence of the distance attenuation coefficient on magnitude was also considered by Lastrico (1970).

Acceleration attenuation curves obtained by other investigators are summarized in Table 1 and compared graphically for a shallow focal depth

magnitude 6.5 earthquake such as the 1971 San Fernando earthquake in Figure

6. All the attenuation relationships summarized in Table 1 are based principally on California data with some records from other locations. The applicability of the relationships developed outside of the western portion of the

United States and Canada has not been demonstrated. There is evidence that

the attenuation of motion in eastern North America is much lower. This has

not been discussed extensively in the literature. Milne and Davenport (1969)

in their seismic regi onal i zati on studies of Canada prepared attenuation

relationships for eastern North America from a study of isoseismal maps.

Figure 7, taken from the Milne and Davenport study, illustrates this much

slower attenuation. Recent Japanese strong motion data assembled by the

author also appear to attenuate more slowly. It is not known at this stage

whether the apparent slow attenuation of the Japanese acceleration is a regional phenomenon or a consequence of the site conditions at the location

of the recording instrument. Denham and Small (1971) reported results of a small statistical study of acceleration attenuation based on data obtained

from repeated triggering of one strong-motion instrument located in New Guinea.

Data from this instrument, which was located on 50 meters of soft sedimentary

rock, gave an exponential coefficient for the distance term of 1.1, similar to

that obtained from the western North American data. Studies of attenuation

effects in different seismic areas should be undertaken.

Acceleration maxima are only one parameter of an earthquake and do not

91
TABLE 1

Data Source

1. San Fernando Earthquake

February 9, 1971

2. California Earthquakes

3. California Earthquakes

4. California & Japanese

Earthquakes

ATTENUATION EQUATIONS

Equation

y = 186206 R"l-83

y = 981 yo

1 +

where log y^ = -(b+3) + 0.81m - 0.027m^

b is a site factor

Graphical Presentation

y = 5 10 0.61m - P log R + Q

where P = 1.66 + 3-60

R

Q = 0.167 - ^-83

R T_ = fundamental period of site

Reference

Blume ( 1965)

Housner (1965)

Kanai (1966)

5, Cloud (1963)

6. Cloud (1963) Housner (1962)

7. U.S.C. & G. S.

8. 11 Selected Records

9. 303 Instrumental Values

10. Western U. S. Records

y = 6.77 l.lgl.lm + r2

y = 1230 e0-8m (R+25)~^

log-^Qy = 6.5-2 log-LQ (R'+80)

Graphical Presentation

y = 1300 e°-""(R+25)-^-^

y = 18.9 e°-8m (r2+400)'1

Milne & Davenport

(1969)

Esteva (1970)

Cloud & Perez

(1971)

Schnabel S Seed

(1972)

y is cm/sec^

R is kilometers (distance to causative fault)

R' is miles (cpicentral distance)

h is miles (focal depth) m is magnitude

92
(T) S4N FERNANDO @ BLUMt 1965 @ HOUSNER 1965 @ KANAI 1966

(D HILNE-DAVENPORT 1969 ® ESTEVA 1970 ® CLOUD £. PEREZ 1971 ® SCHNABEL 1972 ® STAT 1ST ICA,L SET EXCLUDING 'SAN FERNANDO

^ 1000

100

FAULT DISTANCE IN KMS

FIGURE 6

ATTENUATION EQUATIONS FOR MAGNITUDE 6.5

COMPARED TO DATA FROM STRONG MOTION STATIONS

RECORDING SAN FERNANDO EARTHQUAKE

FEBRUARY 9. 1971

93
li r

FIGURE 7

INTENSITY VERSUS DISTANCE

GRAPH FOR EASTERN CANADA.

(AFTER MILNE AND DAVENPORT)

94
alone express a measure of damage potential. Other parameters are probably

of more significance. The most importance of these additional parameters are duration, frequency distribution of the seismic motion, the relative intensity

of motion in terms of the peaks (such a factor could be the ratio of the peak

value to the root mean square value), the maximum ground velocity and the maximum displacement. All these parameters must be considered when characterizing an earthquake. The characteristics of these parameters are considered

later. Provided the perspective between these characteristics and acceleration is maintained, peak acceleration can be considered a reasonable parameter

for describing the risk for seismic zoning (Milne and Davenport, 1969).

Regional maps prepared by Milne and Davenport, relating acceleration levels

and return periods have been incorporated in the Canadian seismic design code.

Return Periods and Probabilities

The risk procedures developed by Cornell is readily suitable for coding

on a digital computer. When this risk procedure is applied to the sources

which are modelled in Figure 3 with the attenuation relationship of Equation

1, the return periods listed below in Table 2 are obtained.

Table 2 RETURN PERIODS FOR PEAK ACCELERATIONS

Return Period (Years)

Acceleration Surface Motion Rock

g Competent Soil Moti on

0.05 4 8

0.10 20 30

0.15 50 60

0. 20 100 100

0.25 250 200

0.30 450 300

0.40 2 ,000 700

The second set of figures representing rock accelerations are based on

a different attenuation equation prepared by this writer and currently being

revised. Seed et al, (1970 and Schnabel and Seed, 1972) have also considered

attenuation in rock.

The return periods in Table 2 can be best understood for the risk

decisions when they are expressed as probabilities. If the events are con- sidered to be unrelated and random in time, then a Poisson distribution may

be used to express the probability of an event occurring during the proposed

lifetime of the building. With the long return periods predicted for large

acceleration levels there is considerable additional uncertainty. The

Poisson distribution is unable to represent this. A Bayesian procedure

outlined by Benjamin (1968) allows this uncertainty to be conservatively

included. Where the return period is shorter than the period of the

95
historical record, the uncertainty is reduced and the Bayesian probabilities

become asymptotic to the Poisson values. The probabilities of the various

acceleration levels being exceeded during a 50 year building lifetime at the

surface of firm ground, and on rock, based on the return periods in Table 2 and the Bayesian procedure are illustrated in Figure 8. The low probability

of a single event with a large acceleration maximum occurring and the high

probability of several events with low acceleration occurring are readily

apparent.

From the probability values shown in Figure 8, the acceleration levels

for the Level I and Level II design earthquakes can be selected. The risk

decisions are therefore made with uncertainties represented at a later stage

in the analytical process. This gives a clearer understanding of risk than

the subjective procedure where a source mechanism and magnitude is chosen

without knowledge of the risk level. The latter procedure has had acceptance

because of a reticence to accept statistical procedures. Also, once the basic

decisions are made, the remainder of the analysis proceeds determini sti cal ly.

Illustrative examples of the subjective and probabilistic approaches can be

provided from other areas of engineering. Only recently has the probabilistic

approach been considered extensively. The procedures are not yet widely

taught at engineering schools within the United States.

The choice of the levels of acceleration for design requires a trade-off

between the cost of higher resistance and higher risks of economic loss.

Structures are made earthquake-resistant rather than earthquake-proof, so sound design will suggest some possible economic loss. The Level II earthquake for which design will be essentially elastic will usually be an event

with a probability of about 0.6 of occurring at least once during the economic

lifetime of the structure. The Level I earthquake will have a probability of

between 0.1 and 0.2. On this basis then the anticipated peak accelerations

for the Level I and Level II earthquakes would be 0.40g and 0.15g for the

example conditions. The frequency content of the motion at the rock level

and the ground surface may be considerably modified by the soil profile.

Observational data obtained from the 1971 San Fernando earthquake and shown

on Figure 6 suggest that the rock acceleration maximum is not greatly different from the soil surface maximum acceleration at motion levels of engineering

significance. The general trend of the data suggest that attenuation by the

soil profile occurs at accelerations greater than O.lg and amplification

occurs at lower acceleration levels.

Two approaches can be taken once the design acceleration levels are chosen. Smoothed response spectra (Housner, 1961; Newmark and Hall, 1969)

have been developed by averaging response spectra computed from accelerogram

records. These represent motions for rock or stiff profiles and can be used

for design where the local geologic conditions are appropriate. Modern

design techniques use either a response spectra or an acceleration time

history. Recently efforts have been made to produce artificial time histories which have smooth response spectra. As real earthquakes do not have

smooth response spectra, it is suggested that where acceleration time histories are required for structural design or where the soil profile may be

expected to modify the site response other techniques be used.

III. Local Effects

Site Geology and Soil Profile

The effect of the local geology and site soils on seismic response

(Kanai, 1952; Duke and Leeds, 1962) was first studied as a wave propagation

problem using reflection and refraction based on Snell's laws. Computational

difficulties in the analyses led to development of other methods. A lumpedmass model representation of the soil profile as a vertical shear beam

96
i .0

0.8 0.10 G

0.6

0. 15 G

0.4 0.25 G

0.2 0.30 G

0.40 G

0.0

0.0 1.0 2.0 3.0 4.0 NUMBER OF OCCURRENCES - CUMULATIVE A) BASED ON RETURN PERIODS USING EQUATION 1

5.0

FIGURE 8

ESTIMATED PROBABILITIES FOR 50-YEAR PROJECT LIFE

97
(Penzien et al , 1 964 ) was first used to analyse site response of soft soils.

The shear beam model and wave propagation procedures have both been used to

show the variation of site response on different soil profiles (Herrera and

Rosenblueth, 1965; Seed and Idriss, 1968). The development of an extremely

efficient algorithm for machine computation of Fourier spectra by Cooley and

Tukey (1965) assisted development of the wave propagation analysis. The wave propagation technique appears to be the most efficient procedure for computing

the response of a soil profile to vertically ascending shear waves. Analysis of the site response as a function of only vertically ascending

waves can be shown to be an over-simplification of a complex response problem.

The computed response spectra for horizontal components at a single instrument

location should have similar characteristic shapes with variations reflecting

the random nature of the basic rock motion. For deep soft profiles, the

assumption of vertical wave propagation gives excellent results. There are appreciable and seemingly inexplicable differences in other types of soil

profiles. Vertical acceleration cannot be produced by a vertically ascending

shear wave. If the vertical motion is principally due to vertically ascending

P waves, the ratio of vertical acceleration to horizontal acceleration should

increase with distance due to the higher velocity of the P wave relative to

the S wave and the consequent lower damping. A study of the 1971 San Fernando

acceleration data showed no evidence of this. Trifunac (1969) concluded that

a predominant part of the near field ground shaking may be composed of surface

waves associated with energy propagating horizontal through near surface rock

and soil layers acting as a wave guide. The analytical data presented by

Trifunac (1970) provides an excellent explanation for the non-stationary time

dependence of the intensity and frequency, but does not conclusively show that

the predominant part of the ground shaking is due to surface wave propagation.

Rayleigh and Love wave propagation in layered media have also been studied

analytically (Lysmer, 1970; Lysmer & Drake, 1971). These studies demonstrated

that selective amplification and reduction of specific frequencies is possible

across areas such as large alluvial valleys where the geologic profile is

locally horizontal. The use of a vertical shear wave model is recommended by

this author subject to the realization of its limitations. Allowance for the

inadequacies of the method by incorporation of some variational procedures in

any deterministic response analysis is highly recommended.

Analyses using the shear wave model can be made in either the time or frequency domains. Using Fourier techniques the analyses are most readily

performed in the frequency domain. Complex Fourier spectra of the output

motion can be obtained by multiplication of the Fourier spectra of the input

motion and the profile transfer function. The development of damped response

spectra within the frequency domain is extremely difficult. Therefore site

response analyses using acceleration time histories must be used even where

it is intended that structural design be based solely upon the response

spectra. To eliminate the effects of the randomness of motion within any

individual acceleration time history it is advisable to repeat the response

evaluation with several time histories and obtain an average response spectra.

This has the additional advantage of allowing an estimate of the probable

variations within any one time history when structural analysis is performed

within the time domain.

Input Motion

The acceleration time history used for input to the base of the soil

profile must be selected on the basis of the available knowledge of the

parameters of earthquake motion. These parameters include the duration of

the earthquake, the frequency distribution of the motion with time and the

variation of frequency content with time. Because the collection of recorded

strong motion accelerations is not large, most of these parameters are not

wel 1 understood

.

98
There is no accepted definition of the effective duration of an earthquake for engineering purposes at the present time. For example, Housner

(1961) referred to the duration of strong shaking while Cloud and Perez (1971)

used the time during which the accelerations exceeded certain values. An approach to computing the effective duration of shaking was proposed by Husid,

Medina and Rios (1969). In their approach the digitized acceleration values

are squared and the sum of the values is accumulated. A plot of the accumulated value against time has a shape resembling a cumulative distribution

curve and provides a direct graphical representation of the duration effects.

Two examples of these summations, for Pacoima Dam, 1971,' and El Centre, 1940,

are illustrated in Figure 9. A numerical definition of effective duration

could be taken from the summation by considering the time period containing

some percent, say 90 per cent, of the total cumulative acceleration squared.

Duration values obtained from recent earthquakes, including Japanese earthquakes, were added to the durations listed by Housner (1970). These are shown in Figure 10.

There is not sufficient knowledge at this time of the variation of

frequency content of the motion during an earthquake to include this in

development of earthquakes for structural design. Some studies of the 1971

San Fernando earthquake aftershocks have been made with variations of frequency included. The variation of intensity with time is somewhat better

understood. A simple subdivision of an acceleration time history into three

parts is usually adequate. They are, a short rise time, a stationary midportion and a final section in which the decay of motion is approximately

exponential. Many research workers have made attempts to generate earthquakes

using white noise, stationary Gaussian processes, or non-stationary processes

(for example, Jennings et al . , 1968; Ruiz and Penzien, 1971). Some workers

have used recorded motions to construct rock motion accelerograms (Seed and

Idriss, 1969). This is a somewhat contradictory procedure as a historic

motion which has probably been modified by the profile upon which the recording instrument was located is used as part of the input to another unrelated

soil profile.

It is recommended that several design earthquakes be developed at each

level. A numerical technique using a random number generator and allowing

choice of the appropriate motion parameters is believed to be the most

suitable procedure. A sample motion for a Level II earthquake with an estimated maximum acceleration of 0.15g representative of a magnitude 7 earthquake

approximately 50 kilometers form the closest point on the fault is shown in

Figure 11. This was generated using a modification of the procedure developed

by Ruiz and Penzien.

Following a site investigation the soil profile can be modelled using

measured or estimated soil properties (Seed and Idriss, 1970). The analytical

techniques used to compute the soil response were developed for linear elastic

materials. The elastic properties of soil are nonlinear and very straindependent. A linear approximation is made by using equivalent linear modulus

and damping ratios representing some average strain (Seed and Idriss, 1969;

Dobry, Whitman and Roessett, 1971). The surface motion can then be computed

using either a lumped mass procedure or wave-propagation theory. An output

time history using the wave-propagation theory is shown in Figure 11. The

output time history using the wave propagation theory and incorporating

uncertainties in the measured soil properties in the soil profile transfer

function is shown in Figure 12. Figures 11 and 12 also show the response

spectra of a single input and output motion for several damping levels, the mean output response spectrum obtained using a set of generated input motions,

and one standard deviation bounds about the mean spectrum. The output acceleration time histories or the output response spectra can be used directly by

the structural engineer for design of the building. For most buildings the

effect of soi 1 -s tructure interaction modifying the ground response need not

be considered. Soi 1 -structure interaction becomes a significant factor only

for very rigid shear wall buildings or buried structures such as a nuclear

reactor.
/ PACO IMA - 1971

EL CENTRO - 1940

.00 6.00 12.00 18.00 24.00 30.00

TIME IN SECONDS

FIGURE 9

CUMULATIVE SQUARED

ACCELERATION RELATIONSHIP WITH TIME

(AFTER HUSID.ETAL, 1969)

100
FIGURE 10

RELATIONSHIP BETWEEN MAGNITUDE a DURATION

OF STRONG PHASE OF SHAKING

101
RESPONSE SPECTRA AVERAGE

(SHOWING STATISTICAL SPECTRAL VARIATION ANTICIPATED BETWEEN INDIVIDUAL TIME HISTORIES IN THE ENSEMBLE)

RESPONSE SPECTRA

X",- -

- ' . - V ^^ OAMPNG RATIO

aoi OK) 100 1000 UNDAMPED NATURAL PERIOD (SGCONDS) RESPONSE SPECTRA

TIME - SECONDS ACCELERATION TIME HISTORY

TIME - SECONDS ACCELERATION TIME HISTORY

SURFACE MOTION OUTPUT

(USING INPUT MOTION SHOWN) A

son. UVYEBTHICKNESS m FEET

TOTAL

UNfT WEIGHT PCF

SHEAA

MODULUS

KSF

DAMPING PERCCKT

LOOSE SAND 15 130 403 I5£

DENSE SAND 20 130 589 14.0 VERY DENSE SAND 41 132 735 125 VERY STIFF CLAY le 116 643 13.2 VERY DENSE SAND 16 130 813 11.6 VERY STIFF CLAY 40 112 402 18.5 VERY DENSE SAND 45 130 966 9.8 VERY STIFF CLAY 15 130 530 14.0

BEDROCK

FUNDAMENTAL PERIOD • I.S SECONDS

INPUT MOTION AT BASE OF PROFILE IDEALIZED SOIL PROFILE

(FROM SET OF GENERATED ACCELERATION TIME HISTORIES) FIGURE 11 INPUT AND OUTPUT MOTIONS TO SOIL PROFILE

FIXED PROPERTIES

102
RESPONSE SPECTRA

20 25 30

TIME - SECONDS ACCELERATION TIME HISTORY

SURFACE MOTION OUTPUT

(USING INPUT MOTION SHOWN) A

LAYER

THICKNESS

FEET

DENSE SAND

VERY DENSE SAND

VERY STIFF CLAY

VERY STIFF CLAY

TOTAL

UNIT WEIGHT PCF

DAMPING PERCENT

ACCELERATION TIME HISTORY

INPUT MOTION AT BASE OF PROFILE

(FROM SET OF GENERATED ACCELERATION TIME HISTOniES)

BEDROCK

MEAN SOIL PROPERTIES VARIED TO GIVE A RANGE OF FUNDAMENTAL PERIOD BETWEEN 1.25 AND 1.75 SECONDS IDEALIZED SOIL PROFILE

FIGURE 12

INPUT AND OUTPUT MOTIONS TO SOIL PROFILE

VARIABLE PROPERTIES OVER CONTROLLED RANGE

103
IV. structural Design Codes

Present seismic design codes specify lateral forces to be used in a static design procedure. The physical behavior of a structure during an earthquake is considered only in general terms and risk potential is not

considered at all. The current consensus of many structural engineers is

that the present seismic codes in use in the United States represent only a minimum guideline standard and require considerable revision. As the seismic

codes have developed over a period of time from simple concepts several

important factors have not been included. This has led to some conflict in

the understanding of the philosophy upon which the code is based.

Seismic design forces are but one type of loading which a building must

resist. A rational design code would be expected to be based on probabilistic

concepts and incorporate an approach reflecting systems analysis. The purpose

of this paper is to review the present state-of-the-art and to suggest modest

changes which may improve the understanding of structural design parameters

within the framework of the present codes. Dynamic design procedures, whether

based on modal superposition using response spectra or direct use of acceleration time histories consider many of the factors affecting the response

behavior of buildings, which have not been included in a seismic design code.

Site characteristics and the response of higher vibration modes are examples.

Seismic design codes of several countries (Argentina, Canada, Chile,

Cuba, France, Greece, India, Japan, Mexico and Turkey) include some modification of seismic coefficients in an attempt to allow for the variation of

response of different soil types. This foreign acceptance of site response

response effects is demonstrated by Tezcan (1971). The Uniform Building Code,

the Recommended Lateral Force Requirements of the Structural Engineers

Association of California, and the proposed new seismic code for the City

and County of Los Angeles, contain no recognition of the variation of response

on different types of soil. The need for incorporation of some recognition of

site effects is becoming increasingly important as more and larger buildings

are being constructed on sites with foundation conditions very different from

those envisioned by the original developers of the codes.

Changes in the present code should be implemented to recognize the larger

lateral forces that can be produced for some combinations of soil conditions

and building properties. To be effective the approach must be simple to use,

while representing rationally the complex aspects of site response. The

suggested procedure presented below is no more complicated than the formulation of the other portions of the Uniform Building Code provision for the

base shear coefficient.

It is recommended that the equation for the base shear coefficient be

modified by the insertion of an additional term S representing the interrelationed effects of the soil and the structure. The equation would therefore

become V = ZKSCW with all the present coefficients remaining unchanged.

The factor S should be the product of two separate factors. These are:

1) A factor independent of period, which is dependent only on the average

soil properties in a zone of influence below the foundation approximately

equal to one half of the building width. This factor which is designated

Sq is designed to allow for the possible motion modification with increasing softness of the surface soil and the possible influence of

static settlements upon the reserve strength of the structure. The

factor Sp may be determined from the following table which uses the same subdivisions as Table 29-B in the 1970 edition of the Uniform Building

Code .

104
Type of Soil

Maximum Allowable S Minimum T Pressure (psf) ° Seconds ^

Good - hard clay, medium

dense to dense sands and

gravels, rock of all types

Average - medium stiff clay,

compact sand or gravel

Poor - loose sand or gravel,

soft clay

8000 1.0 0.4

2500 1.25 1.0

1500 1.5 1.0

2) A factor dependent upon period which recognizes that larger lateral

forces may occur when the periods of the ground motion and the building

are close to each other. This requires estimates of T, the fundamental

period of the building, and Tg , the predominant period of the underlying

soil. The soil coefficient may then be obtained by use of the following

rel ati onshi p :

S = Sq (1.25 - 0.25 cos j

-ff) for 0 < ^ 12

S = S for ^ > 2

Where the predominant ground period has not been established by a detailed site study and the proposed building height does not exceed 150

feet the minimum values for T^ in the table above may be used. The building

period T should be the estimated period of the completed structure including

any soil structure interaction effects.

The 1967 edition of the Chilean Earthquake code modifies the seismic

coefficient C to include the relationship between the ground and structure

periods. A replacement of the existing factor C was also recommended by

Seed, et al , ( 1 970). The existing C parameter is probably low, especially

in the period range less than 1 second and some separate upward adjustment

may also be necessary. Direct replacement of the C parameter should be done

with recognition that the present relationship is based upon response spectra

studies which show that the energy distribution within an earthquake decreases

with increasing period. In Figure 13 the maximum and minimum bounds of the

product of the proposed soil factor and the C coefficient are shown for a ground period of 1 second. The present C coefficient and the Chilean coefficient are shown as dashed lines on the figure. The trapezoidal shape for the

C coefficient suggested by Seed et al , which is shown as a dotted line has

been normalized downward to a maximum of 0.1. All of the code modifications

representing site response effects require either the measurement or estimation of the predominant period of the building site. Within the present

state-of-the-art, it is believed that this ground period can be estimated

with about the same accuracy as the fundamental period of a completed building structure.

V. Conclusions and Recommendations

The types of structures being built today are frequently larger and use different construction techniques and materials than structures which have

survived earthquakes and served as guides for the developers of seismic

design codes. Structures are now being built on a wide variety of sites.

Code forces envisage that the major response of a structure will be in the

105

494-610 O - 73 - 8
.20

GROUND PERIOD = 1.0 SECOND

.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

PERIOD T (SECONDS)

FIGURE 13

SUGGESTED LATERAL FORCE COEFFICIENT

TO INCLUDE EFFECTS OF

SOIL PROFILE AND SOIL- STRUCTURE INTERACTION

106
first mode of vibration. This is not always so. Listed below are four separate areas where it is believed that a con- certed group effort aided by some research could produce improvements in the

engineering field quite rapidly. These are:

1) Levels of motion to be used for design. This is possibly an interdisciplinary communication problem. Single motion pulses

may produce large sei smol ogi cal observations that are not of

practical engineering significance. A rational series of

design basis motion parameters should be developed. These may

use, but should not be confined to, parameters widely used at

present

.

2) The treatment of seismic risk as a relative factor based on probability of occurrence. This may require more emphasis in

studying regional tectonics to allow more objective interpretation of historical sei smi city.

3) The recognition that site conditions be considered in

modifications of the seismic code. Some first step such as suggested herein to include site effects should be undertaken

as soon as possible.

4) A direct corollary of item 3 is the need to include multiplemode response in seismic design codes. The combination of

high rise buildings on developed sites frequently produces

predicted response patterns that cannot be represented by

existing design codes. It must be realized that a design code

cannot adequately cover unusual or complex structures. In such

circumstances it is suggested that building officials should

establish advisory boards who would review and approve the

necessary analytical techniques.

There is a wide gap between the practice of enlightened structural

engineers and the practice presented in code procedures. Although it is

impossible to prescribe a substitute for sound engineering judgment it is

still believed that steps toward bridging this chasm could be a major

achievement of this workshop.

VI. Acknowledgements

The soil structure response factor suggested for code use has resulted

from extensive discussions with Professors Joseph Penzien of the University

of California at Berkeley and Robert V. Whitman of Massachusetts Institute

of Technology. The assistance of the writer's colleagues, Jogeshwar P. Singh and Julio E. Valera in the preparation of this review is gratefully

acknowledged.

VII. References

Ang, Alfredo H. L. and Bruce R. Ellingwood, (1971) "Critical Analysis of

Reliability Principles Relative to Design." Proceedings of the First

International Conference on Applications of Statistics and Probability

to Soil and Structural Engineering, Hong Kong, pp. 2-15,

Barazangi , M. and J. Dorman (1969) "World Seismicity Maps Compiled from

ESSA, Coast and Geodetic Survey, Epicenter Data, 1961-1967" Bulletin

of the Sei smol ogi cal Society of America, Vol. 59, No. 1, February

pp. 369-380.

107
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at Lima, Peru", Bulletin of the Sei smol ogi cal Society of America,

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the Sei smol ogi cal Society of America, Vol. 58, No. 5, pp. 1583-1606,

Octobe r

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Seismic Risk", Proceedings Fourth World Confei^ence on Earthquake

Engineering, Santiago, Chile, A-1 , pp. 69-83.

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Mineral Resources, Canberra, Bulletin of the New Zealand Society

for Earthquake Engineering, Vol. 4, No. 1, pp. 15-30.

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of the Third World Conference on Earthquake Engineering, New Zealand,

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.

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the Causative Fault", Proceedings - Third World Conference on Earthquake Engineering, New Zealand, 94-111.

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.

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.

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109
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, Proceedings of the Third World Conference

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.

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110
National Bureau of Standards Building Science Series 46, Building Practices for Disaster Mitigation , Proceedings

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m
THE PROBLEM OF SEISMIC ZONING

by

S. T. Al germi ssen

Seismic zoning maps for the United States are a relatively recent

development in sei smol ogi cal and engineering research. The first zoning man

of the entire United States was prepared by Franklin P. Ulrich [1] and publisted in 1948. John R. Freeman [2], however, refers to a number of maps

that were prepared for various areas and cities in the early 1900's. These

maps considered some particular aspect of the earthquake hazard problem.

Freeman noted that the chief engineer of the Spring Valley Water Company

sketched a map of San Francisco "long before the great earthquake of 1906"

which considered the possibility of breakage of distribution pipes within

the city during an earthquake. Freeman also includes in his book a map of

"probable relative stability of ground" in Boston during an earthquake. In general, zoning of foreign countries with earthquake problems, such as Japan

and the Soviet Union, was begun at an earlier date than in the United States.

Medvedev [3] provides a summary of early zoning methods.

The purpose of this paper is to outline the factors I believe to be of

most importance in seismic zoning and to discuss the difficulties in estimating these factors and how these difficulties affect the final zoning map.

A Zoning Map Concept

First, it seems desirable to consider what form a zoning map or maps

should assume. The effects of earthquakes can be divided into three categories: (1) those effects which are the result of a certain intensity of

ground shaking; (2) those effects resulting from faulting associated with

the mechanism of the earthquake; and (3) those effects resulting from the

generation of a seismic sea wave or tsunami. All geological effects such as landslides, 1 i q u i f acti on , slumping, etc., occur because of some particular

physical condition of the materials involved, but they all are triggered or activated by a particular level of ground shaking. Thus, it seems desirable,

at least in theory, to prepare zoning maps by displaying the expected maximum

intensity of shaking in a specified time interval together with the areas where surface faulting might be expected with an estimate of the return

periods of the faulting.

Estimates of geological risks such as landsliding, etc., can be made

at a particular site or in a particular region based upon the maximum intensity of shaking (taken from the zoning map) together with an evaluation of

available geologic and engineering maps depicting the hazard or through

geological and engineering investigation of the particular site in question.

Since the intensity of shaking displayed on the zone map would necessarily

have to be given with reference to a specific type of surface material,

corrections would have to be made at each site for the effect of the particular material actually beneath the site.

It is probable that no single measure of intensity of shaking will be

optimal for the estimation of all possible geologic hazards at a site or

Director, Sei smol og i cal Research Group, Environmental Research Laboratories,

National Oceanic and Atmospheric Administration.

112
optimal for use in the design of earthquake resistant structures of all

types. Thus, it is likely that a number of zone maps will ultimately have

to be prepared, each describing intensity of shaking in a way most appropriate

for the use it is intended.

Most zoning maps in the past have been based upon a mapping of Modified

Mercalli (or some other) intensity. Clearly it is desirable to map a quantity that can be more easily interpreted in terms of engineering design than

intensity, but there is considerable question as to how this is best to be

accomplished. It is suggested, at least as an initial map, that ground

particle velocity be used as the parameter to be mapped since there is

appreciable evidence that this quantity is significant for design considerations for a fairly wide range of types of structures.

What are the principle elements involved in the construction of a zoning

map and what problems are encountered in estimating the significance of these

elements in zoning? The elements involved can be roughly divided into

estimates of seismicity and fault breakage, the nature of the seismic source,

seismic wave attenuation, and site response.

Estimates of Seismicity and Fault Breakage

Seismicity is commonly presented in the form of hypocenter maps together

with empirical relations of the forms:

log N = a - bM . . (1 ) where N is the number of earthquakes of magnitude M or greater and a and b are constants to be determined for the region in question, A conversion

from lo (maximum intensity) to M must be made for the older earthquakes for

which instrumental magnitudes are not available so that effective use may be

made of the total historical record of earthquakes, thus improving the

statistical sample and estimates of seismicity. The fit of the data to

equation (1) generally improves as the size of the earthquake sample increases, but may be very poor for small areas of moderate seismicity. Attempts

have been made to represent the occurrence of earthquakes as a Poisson process but this assumption has been seriously questioned [4, 5]

.

It is not known whether equation (1) is valid for very small M because

the networks of seismographs are inadequate to detect and locate small

shocks. It is commonly not understood, for example, that earthquakes as large as M=4 can occur in some parts of the United States at the present time

which are not located [6]. For some large M, equation (1) does not hold

since there is an upper magnitude limit for shocks in each area which depends

on the properties of the crust and upper mantle. It is, therefore, inadvisable to estimate the recurrence of large earthquakes by extrapolation of M from (1) beyond the range of M actually observed. Geological investigations

of the extent of faulting observed to have occurred in recent geological time

may be very valuable in estimating the largest shock likely to occur in a region in the future.

It is a common opinion that not much new information can be obtained

from further study of the historical record of earthquakes. This is a misconception easily demonstrated by examination of a number of recent studies

such as the one reported by Nuttli [7] for the Mississippi Valley. In fact,

great improvement in earthquake statistics would likely result from a very

thorough study of the historical records. Other improvements could result

from expansion of the domestic network of seismograph stations and through

additional studies of microearthquake activity.

Reasonable estimates of probable length of faulting can be made on

113
certain faults, notably the San Andreas in California, but much additional

work needs to be done. Trenching across suspected active fault zones and age

dating techniques promise to greatly expand the amount of information currently available concerning active earthquake areas.

In summary, the statistical sample of earthquakes available throughout

the United States varies considerably in completeness. It should be used in

conjunction with geological evidence of recent faulting and geodetic data

whenever possible to establish the best possible recurrence rates for earthquakes.

The Seismic Source

The nature of the seismic source or mechanism introduces two principle

complications into the zoning problem: (1) some earthquakes appear to be

multiple shocks while others are a single release of energy; and (2) earthquake energy is rarely released at a point source but is released in faulting

which may extend horizontally and vertically for considerable distance.

Multiple event sequences have been postulated for the 1940 El Centro earthquake [8] and for the 1964 Alaska earthquake [9]. A number of questions

remain to be answered. Are all earthquakes above a certain magnitude threshold multiple sequences or do multiple sequences only occur in certain regions?

If multiple sequences occur only in certain regions, which regions produce

these types of earthquakes? Multiple sequences tend to significantly prolong

the intensity of strong shaking. Since only a very small number of earthquakes

have been investigated for multiple events, it is uncertain whether earthquakes

for which strong motion data exist are multiple or single events. Obviously

this greatly complicates the analysis of strong motion data. If data from

accelerograms recorded in an area where multiple sequence earthquakes predominate are used for design purposes in other areas where the earthquakes

tend to be single shocks, considerable errors in estimating the duration of

shaking will occur.

The direction and length of faulting during an earthquake greatly affects

the radiation of seismic waves from the earthquake. Figure 1 shows the general

distribution of maximum horizontal acceleration during the San Fernando earthquake of 1971. While the contours in Figure 1 are only approximate, they do

show a pronounced northwest-southeast acceleration pattern which coincides

with the strike of the preferred fault plane for the earthquake [lOj. The

faulting of the San Fernando earthquake had a significant component of dip

slip motion. For earthquakes in which the faulting is purely strike slip, the

elongation of the acceleration pattern might be even more pronounced. Clearly,

earthquake mechanisms need to be taken into account in seismic zoning. Unfortunately, surface faulting is not common for earthquakes in the United States

outside of California and Nevada and mechanism studies based on instrumental

data have been constructed for only a relatively small number of events.

Mechanism solutions could be computed for a significant number of additional

earthquakes in many areas using seismic data already available and through

calculation of composite mechanism solutions for small earthquakes using data

from portable stations.

Attenuati on

One of the most important factors in seismic zoning is the effect of

attenuation of seismic waves away from the source. At the present time, the

problem of estimating attenuation in the United States can easily be separated

into two parts. Sufficient strong motion data are probably available in

California to reasonably estimate attenuation for zoning purposes. For the

balance of the country almost no acceleration data exist.

1 1 4
115
First, consider the attenuation data available in California. Figure 2 shows a sample of the available data selected to illustrate some of the

current problems. Accelerations recorded during the 1952 Kern County, the

1966 Parkfield, and the 1971 San Fernando earthquakes have been corrected to

bedrock acceleration using a method developed by Campbell [11]. They are plotted in Figure 2,

The correction method developed by Campbell is only an approximate one.

He divides all possible sites into four general categories: crystalline

rock (bedrock); sedimentary rock; shallow alluvium (20-60 feet); and deep

alluvium (60 feet or greater). Accelerations recorded on deep alluvium,

shallow alluvium, and sedimentary rocks must be multiplied by .53, .64 and

.80 respectively to correct them to the acceleration on bedrock. The data

from the 1957 San Francisco earthquake, which has been corrected to bedrock,

was taken from a paper by Idriss and Seed [12].

The 1966 Parkfield and the 1957 San Francisco earthquakes are nearly

the same magnitude and as can be seen from Figure 2, the recorded accelerations compare quite favorably after applying the approximate correction for

site response.

During the San Fernando earthquake, accelerations were recorded at a number of bedrock sites and these are plotted in Figure 2 together with

accelerations recorded at other types of sites which have been corrected to

bedrock. A number of conclusions can be reached from the data in Figure 2:

1. the accelerations, despite some scatter, show a rather regular increase

with increasing earthquake magnitude. There is uncertainty because of lack

of data as to the maximum values of acceleration that might occur near the

causative fault and there is some disagreement among various workers as to

what the maximum accelerations might be. 2. The scatter of the data

increases with increasing magnitude. This is an expected result since the

factors of fault length and departure from a spherical radiation pattern

become more pronounced as magnitude increases. Scatter should be decreased

by introduction of a correction for the focal mechansim. 3. The scatter

of data recorded at "bedrock sites" is considerable and of the same order of

magnitude as for accelerations recorded on other types of materials. This

suggests that more site information, particularly P and S wave velocities

together with a more sophisticated technique for correction to bedrock are needed to further reduce the scatter of the data. 4. Attenuation relationships need to be developed, taking into consideration all of the data

currently available. Corrections for site geology and the mechanism of the

earthquake should be considered in developing the attenuation relationships.

The attenuation formula of Estava [13] plotted in Figure 2 for earthquakes

of magnitudes 5.5 and 6.5 does not fit the data very well. A number of other

published attenuation relationships were also tried with limited success.

The expression for attenuation currently in the literature all seem to be

based on rather limited data; however, a very recent study by Schnabel and

Seed [14] appears to be a comprehensive study of attenuation using the

Cal i f orn i a data

.

Until strong motion ground data become available for other parts of

the United States, attenuation of strong ground motion outside California

will have to be worked out using principally Modified Mercalli intensity

data. One approach is to correlate intensity data available for historical

earthquakes with intensity and attenuation data available for recent earthquakes. Attenuation curves have recently been developed by Nuttli [7, 15]

for 3 to 10 sec. Rayleigh and 1-sec. Lg waves in the eastern United States.

Relationships for the attenuation of surface waves in the United States

can be used to develop a general attenuation curve because at distances

greater than about 60-80 km. surface waves are the predominant type of

ground motion. Very close to the causative fault, the maximum ground motion

is caused by P and S-body waves. The maximum ground motion very near the

116
117
causative fault will be approximately the same as that recorded by strong

motion accel erographs in California. With these constraints (at close

distances and at distances greater than about 60-80 km.) and by use of

Modified Mercalli intensity data (converted to particle velocity, which

appears to be fairly closely related to intensity [16, 17]) satisfactory

attenuation curves can probably be worked out for most areas. Some estimate

of the duration of shaking at moderation distances, greater than about 60

km., from the causative fault can be made by a consideration of surface wave dispersion. Theoretical attenuation curves for California, Nevada, and the

Basin and Range area cannot be used for the rest of the United States.

Figure 3 shows the relative areas shaken at Modified Mercalli intensity VIII

for earthquakes with maximum M.M. intensities of VIII through XI in CaliforniaNevada compared with the eastern United States. The data for California-Nevada

are taken from Algermissen, et al [18] and that for the eastern United States

from Brazee [19]. These data show that the area shaken at intensity VIII in

the eastern United States varies from a factor of about 4 times that in

California-Nevada (for lo = VIII) to as much as 40 times (for lo = XI).

The much larger areas shaken in the eastern United States compared to

the western United States have very significant implications with regard to

seismic zoning. Consider the data in Table 1. In a 100 year period, based

on the historical record, approximately 138 earthquakes with maximum intensities of VII and 40 earthquakes with maximum intensities of VIII have occurred

in the Zone 3 area of California (using the map prepared by Algermissen [20]

shown in Figure 4). In the same period, six earthquakes with maximum intensities of VII and two with maximum intensities of VIII have occurred in the

Zone 3 area centered in southeast Missouri. The ratios of the number of

maximum intensity VIII and VII earthquakes in California in 100 years to the

number of the same size earthquake in southeast Missouri in 100 years are 20

and 23 to 1 respectively (not shown in Table 1). The ratio of the areas shaken in the two areas are 4.5 and 6.4 to 1. If we consider only a portion

of California (in Zone 3) equal in size to the Zone 3 area in southeast

Missouri and reduce the number of earthquakes that occur in the portion of

California in proportion of the area, we obtain the results shown in Table 2.

The ratios of the intensity VIII and VII areas in California and in southeast

Missouri for a 100 year period are reduced to 1.0 and 1.5 respectively.

The conclusion is that if the two areas are compared on the basis of the

total shaken area for a specific time interval the risk is much more nearly

the same than if the number of earthquakes of a particular size are compared.

A similar argument can be advanced for other Zone 3 area in the eastern United

States with more or less success depending upon the reliability of the historical data. This will be developed further in a paper presently being prepared

for publication. Not too much significance should be placed on the specific

ratios presented in Tables 1 and 2 since the ratios of shaken areas vary

considerably with lo as shown in part in Figure 3. The significant point is

that the zones shown on the map shown in Figure 4 do represent equal risk in

a general way when the time period considered is of the order of 100 years

or more and the areas shaken are compared rather than the number of earthquakes that occur in each area.

Site Response

At the present time there are insufficient data available to classify

sites according to the physical properties and thicknesses of the materials

and relate these properties and thicknesses to the expected intensity of

shaking for a range of bedrock intensities. In addition, there is disagreement regarding the behavior of site materials when subjected to strong

shaking [21]. It is doubtful if the problem of site response can be solved

satisfactorily until a number of additional recordings of earthquake strong

motion are obtained at a number of sites (whose properties are well known).

118
100,000

50,000

I 10,000 IT

« 5,000

CO

1 ,000 —

500

Area shaken

(California-Nevada)

Area (Eastern U.S.)

Area (Calif.-Nev.)

100

100

50 5

I .2

o

10 !=

(B

CJ

5 e/5

a»

CO

CO

VIII IX X

Maximum Intensity (Iq)

XI

FIGURE 3. AREA SHAKEN AT MODIFIED MERCALLI VIII FOR EARTHQUAKES

WITH MAXIMUM INTENSITIES OF VIII, IX, X AND XI IN:

(a) THE EASTERN UNITED STATES; AND, (b) CALIFORNIANEVADA. THE RATIOS OF THE AREAS SHAKEN IS ALSO SHOWN.

119
120
Table 1

COMPARISON OF AREA SHAKEN IN CALIFORNIA (ZONE 3)

AND SOUTHEAST MISSOURI (ZONE 3) IN 100 YEARS

2 Ratio

Earthquakes/100 yrs. Area Shaken (km ) Area (Calif.)

lo I Area (S . E . Mo .

)

Area VII VIII VII VIII VII VIII

Cal i f orn i a (Zone 3) 138 . 40 48300 10800

6.4 4.5

S. E. Missouri 6 2 7500 2400

Table 2 COMPARISON OF AREAS SHAKEN IN A PORTION OF

CALIFORNIA AND SOUTHEAST MISSOURI (ZONE 3) IN 100 YEARS

Area

Earthquakes/100 yrs

lo

VII VIII

Area Shaken (km'

lo

VII VIII

Rati 0 (Portion

Area (S

VII

Area

of Cal if

E. Mo.)

VIII

Cal i f orni a Portion of

Cal i f orni a equivalent in area to Zone 3 S. E. Missouri) 32 1 1200 2430

1 .5 1 .0

S. E. Missouri

(Zone 3) 7500 2400

121

494-610 O - 73 - 9
Recordings will have to be obtained at sites over a considerable range of

site excitation to satisfactorily resolve the site response problem.

A Suggested Technique for Zoning

The elements involved in a proposed technique for zoning are illustrated

in Figure 5. Figure 5A shows a hypothetical source area together with historical earthquakes and known faults. Source areas are chosen such that:

(a) they enclose an area of discrete seismicity and insofar as is known,

related tectonic elements. Figure 5B illustrates the log N vs. M relationship

for the source area shown in Figure 5A. The first step is to decide upon a distribution of earthquakes in space in the source area based upon the log N,

M curve. This can be accomplished in a number of ways, two of which are described here. If the relationship between historical seismicity and known

recent faulting and tectonic activity is not well known, the earthquake

activity in the future (based on the log N, M curved derived from historical

data) can be assumed to be equally likely anywhere in the source area in the

future. If the source is divided into n smaller divisions and the number of

earthquakes in the magnitude range AM is ^i\\J\, then the number of earthquakes

likely to occur in the magnitude range AM in each small division or block of

the source area is:

If some particular part of the source area is believed for some geophysical

or geological reason to be more active than other parts of the source area,

the seismic activity in certain of the small divisions of the source can be

increased (weighted) and the activity in other parts of the source decreased.

In this case the slope of the log N, M curve, (b) would remain constant for

the entire source region but the intercept (a) would vary over the area. The

only restriction is that the total number of earthquakes in the range M-j to

M2 in the source area cannot exceed the number predicted in the range of M]

to M2 from the log N, M curve for the source area.

Once the distribution of earthquakes likely to occur in each small

division of the source is decided upon, the effect at each site due to the occurrence of earthquakes in each small division of the source can be com- puted using an attenuation curve such as those shown in Figure 5C. In practice, the distribution of acceleration (velocity or some other parameter)

would be computed for a large number of sites on a grid pattern and the source would also be included in the grid pattern.

From the distribution of acceleration (or some other quantity) at each

site (Figure 5D) it is possible to determine directly the number of times a particular intensity of shaking is likely to occur in a given period of years

at a given site.

The system has a number of advantages, some of which are outlined

bel ow

:

1. The technique allows for incorporating geologic information

concerning earthquake occurrence into the source. For example,

suppose that an active fault is known in the source area but

that historical seismic activity has been largely concentrated

at one end of the fault. Future seismic activity can be

postulated as occurring at (a) the same end of the fault as the historical activity; (b) concentrated at some other

position on the fault, or (c) distributed equally or in some other manner along the fault. The choice would depend upon

the amount of geophysical and geological information available

122
Distance (km.) M

FIGURE 5. ELEMENTS INVOLVED IN PROPOSED ZONING METHOD

123
at the time the map is made and the best current hypothesis

of the nature of earthquake occurrence.

2. The technique does not require the assumption of a specific

distribution or process such as the Poisson process.

3. Programmed for a digital computer, variation of the parameters

involved becomes an easy matter. Variation of parameters is

considered important because it is not obvious at the onset

which parameters will most influence the distribution of

shaking at a particular site. For example, the effect of

changes in the log N, M curve for the source area can easily

be evaluated.

4. The technique allows for the construction of many source areas,

depending upon the amount of geological and se is mo logical data

avai 1 able .

Summary

This paper has discussed some of the problems in preparing suitable

zoning maps and has suggested a technique for the construction of zoning maps

which provide flexibility in the use of seismic and geologic data together

with a quantitative evaluation of the geophysical risk.

References

1. Ulrich, Franklin P., 1948, "Zones of Earthquake Probability in the

United States," Building Standards Monthly , Vol. 17, No. 3, pp. 11-12.

2. Freeman, John R., 1932, " Earthquake Damage and Earthquake Insurance ,"

McGraw-Hill Book Co., Inc., New York, p. 904.

3. Medvedev, S. V., 1965, " Engineering Seismology " (translated from

Russian), U.S. Department of Commerce, Environment Science Service

Administration, p. 260.

4. Knopoff, L., 1964, "The Statistics of Earthquakes in Southern

California," Bull . Seis . Soc. Amer. , Vol. 54, No. 6, pp. 1871 -1873.

•5. Aki , K. , 1956 , "Some Problems of Statistical Seismology," Z i s i n , Vol. 8,

pp. 205-228.

6. Stepp, J. C, W. A. Rinehart and S. T. Algermissen, 1 965 , " Earthquakes

in the United States, 1963-64 and an Evaluation of the Detection

Capability of the United States Seismograph Stations ," ESSA, Coas t and

Geodetic Survey, p. 225.

7. Nuttli, Otto, 1972, "The Mississippi Valley Earthquakes of 1811 and 1812,

Intensities, Ground Motion and Magnitudes," Bui 1 . Sei s . Soc . Amer

.

(in

press )

.

8. Trifunac, M. D. and James F. Brune, 1970, "Complexity of Energy Release

During the Imperial Valley, California, Earthquake of 1940," Bull. Seis . Soc. Amer ., Vol. 60, No. 1, pp. 137-160.

9. Wyss, M. and James F. Brune, 1967, "The Alaska Earthquake of 28 March

1964 ; A Complex Multiple Rupture," Bull . Seis. Soc. Amer. , Vol. 57,

No. 5, pp. 1017-1023.

124
10. Dillinger, W. and A. F. Espinosa, 1971, "Preliminary Fault Plane Solution

for the San Fernando Earthquake," in The San Fernando, California, Earth - quake of February 9, 1971 : A preliminary report published jointly by the

United States Geological Survey and the National Oceanic and Atmospheric

Admini strati on , U.S. Government Printing Office, pp. 142-1 49.

11. Campbell, Kenneth W., 1972, "An Empirical Earthquake Intensity Function

in Bedrock," M . S . Thes i s , University of California at Los Angeles, p. 99.

12. Idriss, I. M. and H. B. Seed, 1968, "An Analysis of Ground Motions

During the 1957 San Francisco Earthquake," Bui 1 . Sei s . Soc . Amer

.

, Vol . 58, No. 6, pp. 2013-2032.

13. Esteva, L., 1969, "Seismic Risk and Seismic Design Decisions," in

Seminar on Seismic Design of Nuclear Power Plants , Cambridge, Mass.,

Massachusetts Institute of Technology Press, pp. 142-182.

14. Schnabel , P. B. and H. B. Seed, 1 972 , "Accelerations in Rock for

Earthquakes in the Western United States," Bui 1 . Sei s . Soc . Amer. . (in press ) .

15. Nuttli, 0. W., 1972, "Seismic Wave Attenuation and Magnitude Relations

for Eastern North America," Jour, of Geoph. Res , (in press)

16. Crandell, F., 1949, "Ground Vibrations Due to Blasting and Its Effects

Upon Structures," Jour. Bost. Soc. Civ. Eng. , Vol. 36, pp. 222-245,

17. Wiggins, J. H., Jr., 1964, "Construction of Strong Motion Response

Spectra From Magnitude and Distance Data," Bull. Sei s . Soc . Amer

.

, Vol. 54, No. 5, Pt. A, pp. 1257-1269.

18. Algermissen, S. T., J. C. Stepp, W. A. Rinehart and E. P. Arnold,

1969, " Studies in Seismicity and Earthquake Damage Statistics, 1969 ,

( Appendi x B

)

, " U . S~! Department of Commerce, ESSA, Coast and Geodeti c Survey, p. 68.

19. Brazee, R. J., 1972, "Attenuation of Modified Mercalli Intensities with

Distance for the United States East of 106°W," Earthquake Notes , Vol.

43, No. 1 , pp. 41-52.

20. Algermissen, S. T. , 1 969 , "Seismic Risk Studies in the United States,"

Proceedings of the Fourth World Conference on Earthquake Engineering , Asociacion Chilena de Sismolgia e Ingenieria Antisismica, Santiago,

Chile, Vol . 1 , pp. 14-27.

21. 1969, Earthquake Engineering Research , A Report to the National Science

Foundation, National Academy of Sciences, Washington, D.C., p. 313.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards, August 2 8-S eptembe r 1,

1972, Boulder , Colorado (Issued February 1973) . 125
WIND HAZARDS FOR BUILDINGS

by

Joseph W. Vellozzi*

and

John J. Healey**

Introducti on

This paper presents a review of the current methodology and criteria for

determining wind loadings on buildings. The objectives of the paper are to:

(1) evaluate practices in current use and identify the best current

practi ces ;

(2) identify priorities for developing improved practices; and

(3) recommend high priority research activities to advance the state

of the art and fill gaps in knowledge.

In the following pages this review is presented in the following order

of topi cs :

(1) Meteorology and Climatology

(2) Drag Loading and Gust-induced Response

(3) Aerodynamics and Aeroel as ti ci ty

(4) Wind Tunnel and Full-scale Investigations

(5) Recommended Research

(6) Bibl iography

While it is not the purpose of this paper to assemble and present detailed wind loading coefficients and other data which are currently being used in

design calculations, this information may be obtained from the references.

Meteorology and Climatology

For the purpose of this discussion, it is convenient to divide the lower

atmosphere into three regions: (1) the surface layer up to about 100 feet;

(2) the "tower layer" up to about 500 feet; and (3) the planetary boundary

layer up to about 2000 feet.

The wind profiles in the surface layer are quite well understood over homogeneous terrain. For engineering purposes, the terrain is considered

homogeneous if the upwind fetch is of uniform roughness for a distance of

about 20 times the height in question. Over uniform terrain the variation of

* Associate and Structural Engineer, Ammann & Whitney, Consulting Engineers

New York.

**Structural Engineer, Ammann & Whitney, Consulting Engineers, New York.

1 26
wind speed with height can be completely described by the roughness length of

the terrain, the friction velocity or drag coefficient and a quantity related

to the vertical distribution of temperature, such as the Richardson Number.

The actual equations, [1]* however, though well understood, are very complex

and the current practice is to describe the variation of wind speed with

height in the surface layer by a power law equation of the form,

V/V^ = {1/1^)^

where V and V-, are the wind speeds at heights Z and Z] and P is an exponent

which depends chiefly upon the roughness of the terrain and to a lesser

extent upon temperature and wind speed. This exponent can be estimated quite

well from meteorological theory. For example, in strong winds the effects of

temperature and wind speed on P are negligible and P is equal to the reciprocal

of Ln (Z/Zq) where Z is the geometric mean height over which the power law is

to be applied and Zq is the roughness length which is typically a few inches over flat open country, 1-2 feet over built-up suburban areas and 10 feet or greater in downtown areas. The exponent tends to be smaller in unstable air

where the temperature decreases rapidly with height compared to the exponent

for stable, strong wind conditions.

It has been observed that wind speeds over the surface layer are greater

than those obtained by extrapolating vertically the power law formula for the

surface layer. It is known that the Coriolis force begins to influence the

wind profile above the surface layer and the general form of the equations

describing this phenomenon has been determined [1, 2]. However, the equations

contain several coefficients which are not well known.

Above the tower layer the wind speed will continue to increase gradually

with height up to the gradient height; that is, the height at which the

effect of surface roughness becomes insignificant. Average gradient heights

are known to lie in the range of about 900 feet over flat open country to

about 1500 feet over downtown areas. Above the gradient height the wind speed

can be assumed independent of height, particularly for strong wind conditions.

Also, above the tower layer the wind turns clockwise in the northern hemisphere by about one degree for every 100 feet of elevation.

Although, strictly speaking, the above power law formula is appropriate

only in the surface layer, the best current estimates of the variation of mean wind speed up to the gradient height are based on this simple law. On

the basis of reported surveys of the variation of wind speed with height [3],

typical values of the exponent, P, in current use are 1/7, 1/4.5 and 1/3 for

open country, suburban areas and city centers, respectively; the corresponding

gradient heights are about 900 feet, 1200 feet and 1500 feet.

To account for the effect of terrain roughness on the variation of wind

speed with height, the current practice is to make use of the wind speed at

the gradient height. Unlike airports where routine measurements of basic

wind speeds are taken for statistical analysis, the centers of large cities

and suburban areas have greater surface friction. Hence, mean wind speeds

and velocity pressures in downtown areas will be less than for open country at

the same height. To adjust for this phenomenon, use is made of the fact that

the wind speed at the gradient height is independent of surface roughness.

The power law formula becomes [4], '

where is the mean wind speed at height Z, V30 is a reference or design

wind speed at a reference height of 30 feet at a nearby airport station and

P and Zg are the appropriate power-law exponent and gradient height, respectively, for the terrain in question.

Numbers in brackets refer to the similarly numbered references at the end of

this paper.

127
In the recommendations of the ASCE Task Committee on Wind Forces [5],

the variation of wind speed with height is based on conditions in only open

level country but a distinction is made between coastal areas and inland

areas. For inland areas the 1/7 power law speed is recommended by the ASCE

Task Committee. For coastal areas, however, the task committee recommended

a power law exponent which varies with wind speed. While this recommendation

is based on observations in coastal areas, it is inconsistent with the current

belief that the variation of wind speed with height is dependent chiefly upon

terrain roughness rather than on wind speed or proximity to large bodies of

water, and additional measurements are required to resolve this difference.

Also, there is considerable doubt as to whether simple power law equations are valid under all conditions for estimating the variation of mean wind speed with height in large cities.

Current codes of practice on wind loading such as ANSI [6] do not permit

reductions in velocity pressures due to direct shielding afforded by adjacent

buildings or structures or by terrain features. However, in the new ANSI

Standard the cumulative effect of upwind structures and/or terrain features

on the transition in the wind profile and the attendant reduction in pressures

is permitted as described above.

For the centers of cities direct shielding can result in markedly reduced

loads for certain wind directions. On the other hand, buffeting in the wake

of upwind structures plus channeling of the flow between adjacent buildings

can increase the loads. Current codes including the new ANSI Standard require

that such increases in loading be taken into account, but the magnitudes of

such increases are known only dpproxi matel y

.

Although not explicit, it is not the intention of the new ANSI Standard

to disallow the use of shielding effects obtained on the basis of wind tunnel

tests in which the surrounding structures and terrain features are carefully

modeled provided that any increases in pressure or suction as a result of such

obstructions are considered in design.

Current American practice makes use of the latest National Weather

Service data on extreme fastest-mile wind speeds [7] as a basis for establishing design wind loads. These speeds are measured by recording the time it

takes for a mile of wind to pass a fixed point with an anemometer which makes

an electrical contact with the passage of each mile of wind. The measurements

are obtained at airports and at other open country locations where the exponent

in the power law for wind speed with height is about 1/7. Although observations may be made at different elevations, they are adjusted to the standard

30-foot elevation prior to analysis by means of the 1/7 power law [7].

Since the extreme wind speeds of each year govern the annual maximum

wind load on a structure, only extreme speeds, corrected for instrument

errors, when necessary, are considered in determining design values. The

annual extreme series for each of 138 Weather Service Stations were fitted

by Thorn [7] with a Fi sher-Ti ppett Type 11 extreme value distribution to

establish design values associated with various probabilities or risks of

being exceeded in any given year. The associated mean recurrence intervals

are the reciprocals of these probabilities and each gives the average time

interval in years between the occurrence of all extreme winds exceeding the

design value.

In order to perform statistical analysis of the dynamic response of

structures to atmospheric turbulence, it is necessary to obtain improved

estimates of wind gust spectra and co-spectra. While a limited amount of such

spectra is available, many of the values are still controversial, particularly

with regard to thermal gradients. It appears that wind speed time series at

high speeds exhibit departures from stationarity which may be critical in the

128
analysis of the data. In most applications, wind fluctuations are assumed

to be Gaussian which is approximately the case for strong winds. However,

much of the available data on atmospheric turbulence is for relatively low

wind speeds. Indications are [1, 8] that the character of turbulence may be

quite different at wind speeds affecting the design of structures and much

more data is needed at such speeds.

Since mean wind speeds are known to be lower in the centers of large

cities compared to speeds in flat open country, accurate information on the

transition in the wind speed profile between smooth and rough exposures is

required in order to transfer meteorological data for open exposures to suburban areas and city centers, and to determine the upwind fetch required for

this transition to take place. Only one form of heterogeneous terrain has

been studied to data--a change of surface roughness along a line at right

angles to the air flow. The results are considered reliable for strong

winds but, unfortunately, such simple geometry is rare. In particular, an abrupt change in surface roughness produces an internal boundary layer which

causes a kink in the wind profile. The kink occurs at a height of about one tenth of the distance to the roughness change. "Regional" or micro wind maps

are required in locations where the transition in the wind speed can have a major effect on relative construction costs. These maps should give the

annual probability of exceedance on wind speed as a function of wind direction and should be continually updated as the terrain becomes more heavily

built up.

Drag Loading and Gust-Induced Response

Important problems associated with establishing the wind loading and

dynamic gust response of buildings or structures in the direction of the wind

include: (1) the effects of shielding and channeling plus topographical

features on the free stream wind; (2) the assessment and distinction between

high local pressures on the cladding versus the lower overall pressures for

design of the main frame; (3) the effects of nearby structures on the flow

pattern; and (4) the effect of the building or structure dynamic response

ch arac te ri s ti cs .

A major innovation in recently adopted building codes and standards on wind loading (e.g. the National Building Code of Canada (1970 Edition) and

the ANSI Standard) is the concept of varying load requirements both with the

dynamic properties of the building or structure and with the roughness of the

terrain. With this newly adopted practice the calculation of wind forces

involves the use of three numerical factors: (1) an exposure factor which is

intended to account for the boundary layer flow over the topography or terrain

at the site; (2) a gust response factor which is intended to translate the

relatively complex wind-gust induced dynamic response phenomenon into an equivalent peak static load; and (3) a pressure coefficient for the various

surfaces and for local tributary areas which relates the pressures and suc- tions on the various surfaces or portions of a structure to the dynamic pressure of the design wind speed. The formula for computing the effective

velocity pressure for design purposes is thus written,

= '^Z^'^30 where q^ is the effective velocity pressure at height Z above the ground,

is the exposure factor which depends upon the roughness of the terrain and

height above ground, that is, upon the free stream boundary layer, G is the

gust response factor which depends upon the dynamic response of the building

or structure to turbulence and q^Q is the design velocity pressure at the

reference height (usually 30 feet). The velocity pressure is the computed

force per unit area of surface perpendicular to the wind, representing the

kinetic energy per unit volume of moving air. It may be expressed by the

129
product Ki X q3o> which is influenced only by geographical location, air

density, height above ground, and exposure. The effective velocity pressure,

on the other hand, takes into account the additional effect of turbulence on the dynamic response of the building or structure and is affected by the gust

response factor G. Although power spectral techniques have proved to be powerful tools for

analyzing the dynamic response of structures to random wind loads and for

formulating gust response factors for inclusion in building codes, standards

and handbooks, the application of these techniques in the estimation of gust

response factors has resulted in major differences between the results obtained among several investigators [4, 9, 10]. The major reason for these differences is the area over which the gust loading is correlated for the structure

as a whole. For example, Vellozzi and Cohen [4] correlate the gust pressures

over the entire surface of a building or structure and arrive at somewhat

lower gust response factors compared to the results of Davenport [10] and

Vickery [9], who correlate the flow only over the windward face. A resolution

of these differences requires a better understanding of the mechanism relating

free-field wind fluctuations to the fluctuating loads on the windward and

leeward faces of a structure. Although wind tunnel tests in boundary layer

wind tunnels [11, 12, 13, 14, 15, 16] have provided significant insight into

this problem, the correlation between such tests and full-scale measurements

is relatively poor and it is recommended that a greater emphasis be placed on full-scale programs to resolve this deficiency.

The general form of the equation for the gust response factor used in

current wind loading practice is given by,

G = 1 + g (o/p)

where a/p is the ratio of the standard deviation of the structure dynamic

response to the mean or average response, and g is a peak factor which relates

the probable peak dynamic response to the root mean square of the response.

It has been shown theoretically that the peak factor can vary over the range

of about 2.5 to 4.0 depending upon the natural frequency and damping ratio for

the building or structure. The peak factor will increase as the natural frequency increases because the number of cycles of vibration in a given period

will increase and the chances will be greater that a given response level will

be exceeded. Consistent with the aerospace industry, an accepted practice is

to use the usual peak factor of 3.0, regardless of frequency. This corresponds to a probability of 0.9973 that the peak value will not be exceeded.

The pending ANSI Standard makes a distinction between gust response

factors for the design of the main frame of buildings with those for the

design of parts and portions such as glass, curtain walls and cladding. This

is because average effective gust pressures tend to increase as the exposed

tributory area decreases. As a consequence, the gust response factors in

current use for parts and portions of structures are somewhat greater than

the gust response factors for the building or structure as a whole.

For ordinary buildings or structures which are usually defined as being

less than 200 feet in height and having a ratio of height to least width less

than 4, the current practice is to use simplified or average gust response

factors which account only for the intensity of the free stream turbulence

plus the size of the building or structure on the correlation of the gust

pressures. However, the structures which are wind sensitive (i.e., greater

than 200 feet in height) and for tall slender structures such as skyscrapers,

towers, stacks and masts having dynamic properties which tend to make them

wind sensitive, a detailed gust loading analysis, in conjunction with wind

tunnel tests in boundary wind tunnels, is generally required. Since the

dynamic response of tall flexible structures to wind loading is critically

dependent upon damping, the accepted practice is to use conservative damping

levels of about one percent for steel structures and two percent for concrete

130
structures, unless properly substantiated test data are available for establishing higher (or lower) values. For cantilevered structures such as tall

buildings and free-standing towers, it has been observed [17] that the

deflected shape of the structure is the same for both the mean wind loading

and for the dynamic gust loading, or stated differently, that the wind loading

does not excite higher modes of vibration. This phenomenon leads to a single

gust response factor for the building or structure as a whole even though the

"intensity" of the gusts in the free stream decreases with height. However,

for structures such as guyed towers and cooling towers, the wind has been

observed to excite higher modes of vibration and a single gust response

factor would not suffice in these situations.

In the case of very large structures, it has been observed [4], at least

theoretically, that the total effective velocity pressure for design purposes

can be less than the fastest mile pressure, which is currently used as a basis

for computing design loads. In other words, a gust response factor which

accounts for the turbulence over the fastest mile of wind can be less than

unity for very large structures because: (1) the fastest mile of wind may not

be of sufficient spatial extent to permit the ultimate pressures to fully

develop on a large structure and (2) the fluctuations in wind speed over the

fastest mile of wind are so irregular and uncorrelated that they are ineffective in producing an increase in the gross loading for large structures.

Nevertheless, current practice does not permit gust response factors which

are less than unity. The effective velocity pressure for a gust response

factor of unity is equal to the fastest mile pressure.

Although the gust response factor increases as the roughness of the

terrain increases, the exposure factor, Kv , and hence the fastest mile windspeed, decreases with an increase in roughness. The decrease in fastest mile

pressure is generally greater than the increase in the gust response factor

and the net result is that the total wind loading is less for a suburban or downtown area than for flat open country by a factor of about 2 to 3. Neglecting wake effects and vorticies shed from nearby, upwind structures, it

follows that the gust-induced vibrations for city structures will generally

be less than for similar structures situated in flat, unobstructed exposures.

However, the percentage of the total deflection, stress, etc. due to gust

loading will usually be greatest for city structures.

An important advantage in treating the wind loading in terms of an exposure factor and a gust response factor in arriving at an effective velocity pressure is that it aids the designer in understanding the dynamic nature

of wind loads and the application of turbulent, boundary layer flows to real

structures. Although the gust response factors and exposure factors in

current use are based on the best information available, a continual measurement program of gust effects and wind speed profiles, particularly for builtup exposures, will be required for a period into the future in order to

continually improve our assessment of gust loads and effects.

The pressure coefficients used in current practice are based largely

upon wind tunnel tests conducted under conditions of smooth, uniform flow

[18, 19, 20, 21] and their validity in extremely turbulent, boundary layer

flows has not yet been completely established. There is evidence [22, 23] to

indicate that suction pressure on leeward walls in urban areas are reduced by

the effects of large scale turbulence. However, the allowances which can be

made for these effects will require additional research on both model and

full-scale structures.

Whereas the practice in the old ASA Standard on wind loading for computing gross loads on buildings was to use a single drag or shape coefficient,

the current ANSI practice is to use consistent external and internal pressure

and suction coefficients and to compute the net wind loading by taking the

algebraic difference between the external and internal pressures. That is.

131
P=qz(Cp-Cp.)

where P is the net wind loading, is the effective velocity pressure, Cp is an external pressure coefficient and Cpi is an internal pressure coefficient.

Internal pressures will not affect the gross drag loads which tend to produce

sliding and over-turning, but can affect the design of the main frame and

walls and must be considered. Consistent pressures and/or suctions for both

the windward and leeward walls plus the roof must be applied simultaneously

in determining the maximum stresses.

With the exception of stack-like structures and signs, it is currently

recognized that drag coefficients for gross wind effects are not adequate for

the complete design of buildings and other enclosed structures. Rather, it

is necessary to have accurate information regarding average and peak pressures

and suctions for each of the various surfaces of buildings in order to perform

a rational design. A general drag coefficient, although adequate for the

study of gross effects, gives no indication of the proportionate wind load on each surface, especially for the side walls. Prior to the new ANSI Standard,

the wind pressure used in the design of the main framing of buildings was generally assumed to act on only the windward wall.

The intensity of local suction pressures on the exterior surfaces of

buildings is known [14, 18, 22] to exceed the mean dynamic pressure of the

approaching wind by factors as high as 5. These high pressures are caused

by one or more of the following mechanisms: (1) action of gusts arriving at

the structure in the free stream wind; (2) buffeting in the wake from an upwind structure; (3) channeling of the flow between adjacent buildings;

(4) reattachment on the structure of flows which have been separated because

of sharp edges, corners, or other architectural features; (5) local vortex

generation by exterior architectural features.

The new ANSI Standard presents local pressure coefficients which are intended to account for these phenomena. The values are based on measurements in both wind tunnels and in full-scale investigations. The new coefficients, in certain cases, are greater than those specified in the earlier

edition of the standard by factors as high as 3 and in other current codes of

practice; however, there have been sufficient structural failures due to wind

forces (e.g. during the recent hurricanes, Camille and Celia) related to

inadequate anchorage of individual panels and roof elements, including glass

breakage, to justify the increased magnitude of the new coefficients.

Relatively little quantitative information exists regarding the effect

of building openings, building volume and layout of internal partitions on the internal pressure changes within the building during strong wind conditions and a great deal of research is needed in this area. The effect of

building openings and building volume on the net loading is significant during

the passage of a tornado. In fact, an effective means of alleviating damage

due to severe pressure gradients during tornadoes is the addition of venting

systems (openings or blow-out panels) to provide a direct release of the

internal pressure during the low pressure phase.

The significant loads to which a structure is subjected to during the

passage of a tornado include the following: direct pressure on the windward

face, outward pressure on the leeward face due to pressure deficit, outward

pressure on the sides, roof and leeward face resulting from the low pressure

in the vortex, and forces due to missiles striking the structure.

Aerodynamics and Aeroel asticity

The chief aerodynamic and aeroelastic problems in the present context

center about the mechanical and fluid instabilities associated with the flow

132
around bluff bodies, since the ground-based structures in question are usually

not streamlined or designed primarily for aerodynamic lift or efficiency.

Flow-induced mechanical oscillations and flow instabilities are the focal

points of interest here. Associated with flow-induced mechanical oscillations

are the numerous galloping and flutter phenomena, whereas flow instabilities

are associated with the velocity and pressure fluctuations occurring in vortex

shedding, flow through screens, edge flow instabilities, etc. While the above

phenomena are understood reasonably well for smooth uniform free-stream flow,

the phenomena for turbulent, boundary layer flow occurring in nature are not

well understood.

Since the flow around bluff buildings and structures is subsonic and

essentially incompressible, the influence of the wake conditions upon the

gross wind forces is great and of paramount importance towards an understanding of the flow phenomena. The effect of the Reynolds number upon wake flow

and wake-body interaction is not well explored so that the ranges of validity

of the results of wind-tunnel tests become an important consideration. There

is a need for Reynolds number effects to be evaluated in terms of both the

local and gross flow patterns around bluff bodies in order to give physical

insight into flow instabilities.

While the study of the flow over a circular cylinder is still a formidable problem, the study of flow over various bluff bodies of sharp-edged

cross section may well prove to be of equal, if not greater, importance

towards as improved understanding of structural wind problems.

Because of the relative analytical intractability to date of flow around

bluff bodies, suitable analytical models of instability for such bodies are virtually non-existent. However, the use of potential flow models of bluff

bodies in the presence of wake vorticies [23] appears to be of some promise,

and, generally, the judicious use of computer models of the flow can provide

insight into certain instability phenomena although this process is time

consuming and relatively costly. Consequently, experimental research will

probably remain as the area of interest for a period into the future. It will

be of importance in this area for the experimental aerodyn ami c i st to observe

and record key flow characteristics which provide a broad understanding of

the phenomena. The need also exists for identifying and classifying those

flow phenomena (such as separation phenomena) which are present in the various

classes of instability so that an ordered picture can be established of the

aeroelastic phenomena of bluff bodies. Recent research [24, 25] has been

concerned with identifying the non-linear phenomena of galloping and vortexinduced oscillations of bluff bodies and it seems promising that sound dynamical models of these oscillations, with the unsteady aerodynamic forces as empirical inputs, can be established theoretically.

Those classes of instability which can occur for streamlined structures

(such as flutter in coupled torsion and translation modes) presently require

somewhat less attention than the bluff-body problem because of the strong

emphasis already given to them by aeronautical engineers. They are nevertheless of fundamental importance as a background towards an improved understanding of bluff-body aerodynamics.

The effect of upstream turbulence upon the flow over bluff bodies is of

fundamental concern. To simulate this phenomena, it becomes necessary to

establish experimental methods for adequate representation of the turbulent

boundary layer flow occurring in natural winds. It has been observed that

turbulence in the free stream can delay the onset of certain aeroelastic

instabilities such as flutter and in certain cases can practically eliminate

them due to turbulence-induced changes in the flow over the body or due to

reduced correlation between random flow effects at separated points on the

body

.

Continued development of representative test and simulation methods, so

1 33
that similarity to full scale in enhanced, is necessary with regard to forecasting the performance of complex structures in natural wind environments.

Such methods are presently the last effective resort in particular cases where existing analytical methods cannot follow the complexities of the

actual full-scale environment.

Wind Tunnel and Full-Scale Investigations

Because of the complexity of turbulent boundary layer winds in the

atmosphere, it is unlikely that quantitative solutions of the wind loading

and dynamic response phenomena for real structures can be obtained without

the aid of wind tunnel tests. Moreover, for proposed structures of taller

and lighter construction and having higher strength materials and lower

overall safety factors compared to "traditional" designs, modeling in a properly simulated wind environment produced in the laboratory may be the

most economical if not the only way to obtain the required information.

The scaling criteria required to achieve dynamic similitude between

model and full scale are well known [24]; however, these criteria are extremely difficult to satisfy, even when special purpose wind tunnels are used. The best practice is to adhere to the following criteria, whenever

possible, in wind tunnel investigations: (1) Full scale characteristics of

turbulent boundary layer flow including mean wind speed profile, distributions

of scale and intensity of turbulence, and thermal gradients should be simulated; (2) Adjacent structures and terrain features which influence the flow

should be modeled; (3) Where dynamic response of the building or structure

is important, suitably scaled aeroelastic models should be used.

In general, boundary layer type wind tunnels, such as those at the

University of Western Ontario and at Colorado State University, in which the

floor of the tunnel is roughened artifically to simulate natural terrain, are highly desirable for simulating natural winds. However, other methods of

generating turbulence have been found to be successful including the use of

graduated grids and screens, air jets, shutters and vortex generators [14].

There is need to establish correlations between models and full-scale

measurements, particularly with regard to the difference in Reynolds number.

All too often, however, the results of such tests are considered proprietary

and are not generally available to the public.

Although the Reynolds number for the model scale is typically only

1/300 to 1/500 of full scale values, the results for sharp-edged models are considered reliable because of the fixed position of the flow separation.

This is not the case, however, for rounded structures such as stacks and

cooling towers, and more full-scale studies should be conducted on such

structures

.

The Commonwealth Advisory Aeronautical Research Committee has proposed

a round robin of tests using a standard prismatic model to aid researchers in

evaluating results obtained from different wind tunnels. This model was scaled from an actual building on which surface pressures have been measured

so that the different wind tunnel test results can also be correlated with

the full-scale measurements. Such a program affords an excellent opportunity

to fully evaluate all aspects of the flow simulation including effects of

Reynolds number and wind tunnel blockage, and would be a worthwhile undertaking in the United States.

In addition to the need to simulate the characteristics of turbulent,

boundary layer winds, tests to determine the dynamic response of structures

to gusty winds necessitate that suitably scaled aeroelastic models be used.

A major limitation is the availability of modeling materials with the required

1 34
physical properties. In the case of tall slender structures where the

dynamic response due to wind gusts is predominately in the first mode of

vibration, one approach [14] has been to use a rigid model which is permitted

to rotate about its base on a gimbal. The gimbal, in turn, makes use of

springs and electro-magnetic dampers to simulate the modal stiffness and

damping of the full scale structure. The assumption here is that the fullscale structure deflects in essentially a straight line which is approximately

the case only for framed buildings with shear wall cores. The errors that

result when the predominant mode of vibration is not a straight line should be

explored. Since the modal damping is not generally known for unique structures, a range of probable damping coefficients is covered in the test program

to identify worst-case conditions.

While the use of spring mounted rigid body models appears to be satifactory for many structures, this technique is not appropriate when the critical

mode of vibration is not known a priori or when the response occurs in several

modes of vibration. Shell structures such as cooling towers and membrane type

roof structures, plus suspension bridges, are examples in which particular

attention must be paid to aeroelastic modeling in order to obtain meaningful

results. For suspension bridges, it has been found that the results from

tests on aeroelastic models of the full bridge always present a more favorable

picture than tests on spring-supported section models [13].

Full-scale investigations of structures is an area in which considerable

effort has been expended within the past four years. For these investigations

various types of instrumentation and measurement techniques are being developed. Hot film anemometers and Doppler radars are presently being used in

atmospheric studies but they are still in a state of development. Concurrent

with this development there is a need to continually improve pressure transducers and to develop more reliable laser devices and tilt-meters to record

slowly varying drifts of buildings. The problem of obtaining adequate field

data on mean velocity profiles, particularly in urban areas cannot be over- emphasized. Much useful information on wind characteristics already exists

and should be compiled. To accomplish this, a task committee (perhaps an ASCE Committee) should be established to assemble information on wind characteristics as it becomes available and to coordinate new research programs.

Because there is an increasing interest in the total performance of

structures there appears to be little doubt that Monte Carlo methods will

eventually be applied to the wind loading problem in structural design. For

this purpose magnetic tapes of detailed observations will be needed, especially under critical conditions. In addition, meteorologists devote most of

their efforts at present to real time or forecasting problems which are of

little value in the planning and design of future structures. More effort

should be devoted to the non-real time or cl imatol ogi cal problems with special

emphasis on the internal characteristics of storms.

With regard to full-scale investigations of wind forces on buildings and

structures, it is recommended that companion wind tunnel tests be carried

out whenever possible. This recommendation is emphasized for rounded structures where the effect of differences between model and full scale Reynolds

numbers is presently unknown.

While the primary motivation of full-scale investigations has been the

correlation with wind tunnel tests, information necessary to establish distributions of extreme loads should also be obtained from these investigations.

Such studies should establish as a minimum the relationship between the wind

speed fluctuations in the free flow and the pressure fluctuations on the

structure. Supplementary measurements of the dynamic response should be

correlated with the wind speed and pressure data using spectral techniques.

In addition to the data obtained from elaborate instrumentation set-ups

on specific structures, there is a need to obtain long-term field data

135
involving minimal instrumentation of several structures within a given region.

Recommended Research

As a final summary, research activities which should be given priority

are listed below:

1. - Long term measurements should be taken on full-scale structures

for the purpose of establishing distributions of extreme loads,

effects of repeated loads and damping ratios.

2. - An urgent effort should be made to establish correlations between

model and full-scale measurements.

3. - A task committee should be formed to compile existing wind data

relevant to structural design and to coordinate future wind

resea rch programs

,

4. - A standard aeroelastic model of a full-scale building should be

tested in various wind tunnels to afford an evaluation of the

different methods of simulating turbulent boundary layer winds.

The results should also be compared with measurements on the

prototype

.

5. - Meteorologists should be encouraged to devote more effort to the

non-real time or cl imatol ogi cal problems with special emphasis on the internal characteristics of storms.

6. - A meteorological survey report should be prepared which summarizes

the existing meteorological and cl i matol ogi cal data which is

relevant to structural design.

References

1. Panofsky, H., "The Structure of Atmospheric Shear Flows," Proceedings , Fluid Dynamics Panel Specialists' Meeting on "Aerodynamics of Atmospheric Shear Flows," Munich, N.A.L.L). Advisory Group for Aerospace

Research and Development, 1969.

2. Brooks, C. E. P., Curst, C. S. and Carruthers, N., "Upper Winds Over

the World-Part 1, The Frequency Distribution of Winds at a Point in

the Free Air," Quarterly Journal , Royal Meteorological Society, 72,

London, 1946.

3. Davenport, A. G., "A Rationale for the Determination of Design Wind

Velocities," Journal of the Structural Division , ASCE, Vol. 86, No.

ST5, May 1960.

4. Vellozzi, J. W. and Cohen, E., "Gust Response Factors," Journal of the

Structural Division , ASCE, Vol. 94, No. ST6, June 1968.

5. ASCE, Final Report of the Task Committee on Wind Forces, Transactions , Paper No. 3269 , Vol . 126 , 1 961 .

5. American National Standards Institute, "Building Code Requirements for

Minimum Design Loads in Buildings and Other Structures," Latest Edition.

7. Thorn, H. C. S., "New Distributions of Extreme Winds in the United

136
states," Journal of the Structural Division , ASCE, Vol. 94, No. ST7,

July 1968.

8. Harris, R. 0., "The Nature of the Wind," Proceedi ngs , Seminar on "The

Modern Design of Wind-Sensitive Structures," CIRIA, ICE, London, 1970.

9. Vickery, B. J., "On the Reliability of Gust Loading Factors,"

Proceedi ngs , Technical Meeting on "Wind Loads on Buildings and

Structures," National Bureau of Standards, January 1969.

10. Davenport, A. G. , "Gust Loading Factors," Journal of the Structural

Division , ASCE, Vol. 93, No. ST3, March 1 967.

11. McKeon, R. J. and Melbourne, W. H., "Wind Tunnel Blockage Effects and

Drag on Bluff Bodies in a Rough Wall Boundary Layer," Proceedi ngs , Third International Conference on Wind Effects on Buildings and

Structures, Tokyo, 1971.

12. Scruton, C, and Rogers, E. W. E., "Steady and Unsteady Wind Loadings

of Buildings and Structures," Phil. Transacti ons , Royal Society, London,

A, 269 , pp. 353-383 , 1971 .

13. Scruton, C, "The Use of Wind Tunnels in Industrial Aerodynamic

Research," Advisory Group for Aeronautical Research and Development,

Report 309, October 1960.

14. Cermak, J. E. and Sadeh , W. Z., "Wind-Tunnel Simulation of Wind Loading

on Structures," ASCE, National Meeting on Structural Engineering,

Baltimore, Maryland, Conf. Preprint 1417, April 1971.

15. Baines, W. D. , "Effects of Velocity Distribution on Wind Loads and Flow

Patterns on Buildings," Proceedi ngs , Symposium on "Wind Effects on Buildings and Structures," Vol. 1, National Physical Laboratory,

Teddington, 1963.

16. Bearman, P. W., "Wind Loads on Structures in Turbulent Flow,"

Proceedi ngs , Seminar on "The Modern Design of Wind-Sensitive Structures,"

CIRIA, ICE, London, 1970.

17. Davenport, A. G., "The Treatment of Wind Loading on Tall Buildings,"

Proceedi ngs , Symposium on Tall Buildings, Southampton, Pergamon Press,

T96F:

18. Ching, N., et al , "Wind Tunnel Studies of Pressure Distribution on Elementary Building Forms," Iowa Institute for Hydraulic Research,

State University of Iowa, 1951.

19. Holdredge, E. S. and Reed, B., "Pressure Distribution on Buildings,"

Texas Engineering Experiment Station, College Station, Texas, 1956.

20. Normen fur die Bel astungsannahmen , die Statischen Berechunungen , die

Abnahme, die Ueberwachung , und den Unterhalt der Bauten, SIA No. 160,

Ing. und Arch. Verein, Zurich, 1956.

21. Delaney, N. K. and Sorensen, N. E., "Low Speed Drag of Cylinders of

Various Shapes," NACA Technical Note 3038, 1953.

22. Jensen, M. and Franck, N., "Model-Scale Tests in Turbulent Wind,"

Part 11, The Danish Technical Press, Copenhagen, 1965.

23. Roshko, A., "A Review of Concepts in Separated Flow," Proceedi ngs , Canadian Congress of Applied Mechanics, Vol. 3, Quebec, 1967.

137

494-610 O - 73 - 10
24 Whitbread, R. E., "Model Simulation of Wind Effects on Structures,"

Proceedi ngs . International Conference on Wind Effects on Buildings and

Structures, H.M.S.O., London, 1963.

25 Wootton, L. R. and Scruton, C, "Aerodynamic Stability," Proceedings

,

Seminar on "The Modern Design of Wind-Sensitive Structures," CIRIA, ICE,

London, 1970.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards , August 28-September 1,

1972, Boulder, Colorado (Issued February 1973).

1 38
LAND USE PLANNING AND NATURAL DISASTER MITIGATION

by

Wil 1 i am J . Petak^

2 Margarita McCoy

3 Walter J. Monasch

4 James E. Slosson

5 Donald F. Moran

John H . Wiggins , Jr.

I . Introduction

The land use planning process consists of a series of surveys, analyses

and policy decisions starting at the general level and ending with specific

regulations regarding the construction, use and function of a building or structure on a specific piece of ground. As such, land use planning encompasses the full range of pol i cy-pl anni ng- i mpl emen tat i on actions of any

governmental unit engaged in disaster mitigation.

Land use planning must be interdisciplinary in nature. Although the

major early contributions came from engineering, architecture and the law, the

disciplines of public administration, political science, economics, geography

and sociology have added new information and understanding. Modern planning

must take into account these new perspectives.

Thus, land use planning must be reviewed as a key component of governmental planning. As a key component, land use planning will be concerned

with the totality of disaster mitigation, and not merely the physical location

of buildings. There must be concern for the older and already developed urban

areas as well as the open and as yet undeveloped rural areas.

A plan can be only a guide to implementation of a program designed to serve the public good. The impact of disasters, such as earthquake or tornado

can only be diminished if the programs and regulations set forth in the plan

are implemented. To be effective in disaster mitigation, land use planning

must successfully integrate the policy process with the implementation program

Only through this successful integration will the public good be served.

Generally, implementation occurs through the enforcement of zoning

regulations, subdivision ordinances and building codes. It is at this point

^Associate Professor of Political Science, California State University,

Ful 1 erton

.

2 Instructor and Assistant Director, Graduate Program - Urban and Regional

Planning, University of Southern California.

3 Planning Director, County of Santa Cruz, California, and Past President,

American Institute of Planners.

4 Consulting Engineering Geologist, James E. Slosson and Associates.

5 Consulting Structural Engineer.

^President, J. H. Wiggins Company.

139
in the planning process that the best of plans may fail. Failure, as such,

is usually manifested in a deviation from the regulation, and is frequently

granted because of political pressure. This pressure can usually be shown to

result from economic concern or from a lack of full comprehension of the risks

involved.

Thus, a major difficulty in the area of land use planning for disaster

mitigation at any governmental level is the significance, largely in economic

terms, of the impact of plans and regulations, and the resultant argument that

the "state-of-the-art" does not warrant such rigid controls. It should be

apparent then that effective policy can only be developed when data and

information are made available by the technical community in such a manner

that all parties (political, administrative and citizen) can comprehend the

impact and risk of various decisions.

Finally, a fundamental question to be addressed in disaster mitigation

is the relative role of the federal, state and local levels of government in

establishing policy which effectively guides urban development. Generally,

it is only when on level of government fails to respond adequately to the

public need that another level of government enters into the process. This

has been fully demonstrated in the area of environmental legislation.

There are indications that review and intervention by higher levels of

government may be forthcoming for local land use planning agencies concerned

with disaster mitigation.

This paper attempts to address each of the above areas. The paper is

divided into seven sections, including the Introduction and a list of recom- mendations for research.

Following the Introduction, there is a discussion of the function of

land use planning in disaster mitigation. The third section presents background on the responsibilities of the levels of both organization and

government in the planning function. Section four discusses various natural

disasters that can be mitigated through effective planning. Current practices

in land use planning which may be used in disaster mitigation are presented in

section five. Finally, an approach to seismic impact assessment for general

planning purposes is presented in section six.

II. Function of Land Use Planning in Disaster Mitigation

Natural disasters may be "acts of God," but unnecessary losses in life,

property, and social disruption often result from improper land use in

disaster-prone areas. Within federal, state and local levels attention is now focusing on providing a wider and more effective range of methods to

mitigate the effects of these disasters. Land use planning and regulation

at the state and local level to guide private and public uses of land in

hazard-prone areas can be one of the important management tools for avoidance

of major losses.

There are, however, some formidable difficulties in making this tool

operationally effective in the application to hazard mitigation. As background to consideration of the problems and the promise of land use planning

as an approach to disaster mitigations, a discussion of the development and

function of planning in government is provided below.

Land Use Planning: Overview

"Since planning is directed primarily toward the end of rational and

systematic decision-making by government, the concerns and scope of planning

should be as broad as the concerns of the government it serves, the activities

140
the government engages in, and the decisions the government must make. The

scope of planning must also relate to administrative devices available, and

the tools at hand to carry out proposals." [1]

Planning, as a separate and identifiable function of government, began

in the cities of this country early in the century. Early city planning was almost totally concerned with the physical aspects of the city. "Land,

people, buildings - throughout its history city planning has been expressed

mostly in these terms. Of the three, the land has given planning its most

distinctive cast. At the core of any planning program are land use studies."

[2]

As the cities grew, the complexity of their problems grew ahead of them.

Cities, which once were clearly delineated from the country that surrounded

them, became instead centers of vast urbanizing areas. Satellite cities,

suburbs, exurbs - slurbs; the metropolis. An ever diminishing portion of

effective city planning could consider only the area within the core city

boundaries. City planning became urban planning to reflect a broader scope.

Broadening further, interstate and intrastate regional planning has received

the support of the Federal government as the need becomes apparent to con- sider urban problems within the context of their areas of influence. And

state planning, once limited to providing an enabling statute for city

planning at the local level, is beginning to show evidence of assuming

responsibility for planning problems properly considered at that level.

Accompanying the wider spatial concepts in planning has been the

realization that physical planning cannot be separate from the other subsystems of the metropolis. As Daniel Patrick Moynihan has said, "the trouble

with urban problems is that everything is related to everything". In response

to this understanding, urban planning has become an interdisciplinary science.

Its techniques and perspective are derived from a variety of disciplines.

Although the major early contributions came from engineering, architecture

and the law, the disciplines of public administration, political science,

economics, geography and sociology have added new information and understanding. The role and function of modern urban planners must take into account

these new perspectives.

Land use planning must, therefore, be viewed as a component of governmental planning. It remains a key component, the target or visible manifestation of most of local and regional governmental planning, as enabled by the

states. As a component of the government planning function, land use planning

is primarily concerned with the physical location of buildings and/or facilities and the use of land in a geographical area. The land use planning process

consists of a series of surveys, analyses and policy decisions which should

begin at the general level and develop toward specific regulations regarding

the construction, use and function of a building or structure on a specific

piece of ground.

Land use planning can be considered as sequential, involving three major

acti vi ti es :

1. Development of general plans.

2. Adoption and administration of zoning regulations

regulations, and building and grading regulations

implementation of the general plans.

subdivision

for

A constant feed-back process in which existing regulations are regularly reviewed to insure that they continue to serve community objectives in the light of changing conditions, new knowledge or evolving perceptions of the public interest.

141
A. General / Comprehens i ve Plan

"The General Plan is the official statement of a municipal legislative

body which sets forth its major policies concerning desirable future physical

development" [3] according to T. J. Kent, Jr., a leading authority on the

General Plan concept. (To avoid confusion, it should be noted the "General

Plan", "Comprehensive Plan" and "Master Plan" are used interchangeably and

often in combination in the literature.)

The general plan is not a law. It is a long range plan, often for 20 or 30 years hence, and thus cannot be interpreted specifically. As an official document it is supported by more detailed, shorter range planning

documents which are law; the zoning ordinance, subdivision regulations, and

grading and building codes.

The planning process which moves from the general policy decisions of

the Comprehensive Plan to the specifics of ordinances and codes should be

used to encompass hazard mitigation into any program of implementation.

Stuart Chapin describes the process thus: "Urban land use policies are - a series of guides to consistent and rational public and private decisions in

the use and development of urban land. They are maxims to guide land

development in principle. We may conceive of the formulation of policies as proceeding from the general to the particular, with each level of policymaking supplying the foundation for subsequent, more detailed policy determinations". [4]

Communities are products of human efforts and, as a result are no better

or worse than the sequences of decisions that shaped them. If planning is

"the process of making rational decisions about future actions directed toward

the attainment of community goals", the planning process must rest upon

research. That is, data and insight gained by analysis must be used in the

development of general policy judgments.

It must be understood that plans are not made in a normless vacuum and

that they are not without normative impact. The problems of physical development and social welfare are related in very complex ways. Specifically, in

the area of disaster mitigation, the protection of society from disasters

(social welfare) is a legitimate goal for proper and effective physical

development regulated through policies of the general plan and specific

ordinances which support it. Therefore, a part of any comprehensive plan

must be "an examination of all existing codes, ordinances and administrative

policies dealing with land use, control and regulation" [5] to ensure that

they effectively support the goals an/ objectives of the general policy

deci s i ons

.

B . Land Use Regulations

Land use regulations such as zoning and subdivision control are the

major instruments of implementation for the general policies of the Master

Plan. The regulations involve legislative and administrative action at the

state and local levels. The power of government to regulate land use on behalf of the public health, safety, and general welfare is considered a sovereign power of the state, which it may delegate to cities and counties.

This power is guaranteed under the Tenth Amendment to the United States

Constitution.

The power of the states to regulate in the best interests of its

citizens is broad but not unlimited. Individual rights are protected by

both the state and federal constitutions, including the right to hold and

use private property against arbitrary, unjust or overzealous legislative

action.

142
state constitutions authorize state legislatures to enact general lav/s.

The legislatures commonly delegate a portion of this legislative power to

local municipal units by enacting "enabling" statutes which authorize municipal legislative bodies to enact local regulations. State and local legislative

bodies also establish administrative agencies to administer and implement

legislative programs.

Administrative agencies usually have no inherent powers except for those

delegated to them by enabling legislation. An attempt to exercise local

power without express authorization and careful compliance with the procedures

of the state law is generally considered invalid. Land use controls such as zoning, subdivision regulation, and building codes have been enacted primarily

at the local level. Procedures specified in state enabling acts, authorizing

adoption of these local regulations such as requirements for notice and hearing, prior planning, voting, publication, and other matters, are mandatory and

must be carefully observed.

Public facilities, those administered by local, state or federal agencies,

are not subject to land use regulation. The recommendation that they be made

subject to these regulations is common but has been ignored so far, with a few specific exceptions.

Land Use Planning for Disaster Mitigation

Land use regulation to reduce the effects of natural hazard shares the same sources of authority and is subject to the same constraints as all

other land use regulation. Its restrictions, placed upon private uses of

land in disaster-prone areas, are developed through the legislative process,

are established in the public interest, and are intended to allocate lands

to their most appropriate uses. Most common among existing regulations for

disaster prone areas are the restrictions placed on land use in flood hazard

areas, often referred to as flood plain zoning.

Regulation of land uses in disaster-prone areas, like all other land use regulation, must meet four general criteria in order to be effective and

valid:

1) Local regulations must be in accord with state statutes

authorizing such regulations.

2) Regulations must serve valid police power objectives.

3) Regulations must demonstrably aid in the accomplishment of

the stated land use and general plan objectives.

4) Regulations must not be discriminatory in content, in application,

or enforcement.

These criteria must be observed in each phase of a valid regulatory

program. A regulatory program of this type may generally involve:

1) Enactment by the state legislature of a statute authorizing local

units and/or a state agency to plan and regulate private uses in

disaster-prone areas. This may be done as part of a broader land

use planning and regulatory effort or more specifically for some

type of disaster common to the area.

2) Selection of protection policies by local agencies for management

of disaster-prone areas, probably as part of a broader land use planning effort.

3) Drafting of local regulations required to implement the desired

143
protection policies.

4) Adoption of regulations,

5) Administration of regulations.

6) Enforcement.

In considering the range of regulatory objectives, it is important to

note that zoning and subdivision controls are utilized to promote the general

social, economic, and political well-being of a community through implementation of a plan for community growth. Each of the control sets is usually

enacted to achieve a wide range of goals and those related to disaster would

be "preventative" in nature.

Building codes, in contrast, are "protective" or "corrective" regulations

and are used to regulate construction through controls over materials, design,

and building size. They are not necessarily implemented in relation to the

general plan.

A . Zoni ng

Zoning as a method of land use control has been used throughout the

country by local units of government to guide private development of land in

a manner consistent with the public health, safety and general welfare. It

is somewhat surprising, however, that land use controls which have been in

wide-spread use have rarely contained control provisions to guide land uses

in hazard or disaster-prone areas. The literature, for example, has recom- mended for many years the use of zoning and other land use controls to compel

adjustment of land uses to the flood hazard, but, of the total communities

affected, few have established effective flood plain controls [6].

Zoning involves the division of a geographical governmental unit into

districts. Within these districts, zoning typically regulates: (1) The

dimensions of buildings and other structures; (2) The area of a lot which

may be occupied and the size of required open spaces; (3) The density of

population; (4) The use of buildings and land for trade, industry, residence

or other purposes [7].

One major characteristic of zoning is that the regulations can differ

from district to district in terms of the items regulated, as well as zoning

definitions applied.

The division of a government unit into districts should be based upon

the general land use plan which was developed to guide the growth of the

community. Many state enabling acts require that zoning regulations be

consistent with the general plan.

Typically, a zoning ordinance will consist of two parts:

(1) A written test which details the regulations applying

to each district, including appropriate administrative

provisions.

(2) A zoning map which specifies the boundaries of districts

for various uses.

Thus, zoning can be used to regulate WHAT the land and buildings may be used

for, WHERE specific uses may be conducted and, HOW uses are to be performed.

In this regard, zoning can be used to establish effective restrictions

regarding land use in certain disaster-prone areas. For example, zoning can prohibit the building of certain structures across an earth fault or prohibit

144
structures from being built in floodways.

B . Subdivision Regulations

Subdivision regulations are enforced through ordinances which prohibit

the filing or recording of a plat showing a division of a tract of land into

smaller parcels, usually building lots, without prior approval of a municipal

planning agency. All states have delegated the power to regulate subdivision

of land to at least a portion of their municipalities. Although the enabling

laws vary widely, most have their common beginnings in the early subdivision

acts drafted by Bettman and Bassett [8].

Subdivision controls appear to be nearly as useful as, or in some instances, even more useful than, zoning in regulating hazard areas. First,

they may be used to discourage land speculation and "victimization" of buyers

of lands by requiring that hazard information be shown on the plat or by

prohibiting subdivision of certain unsuitable lands. Second, they appear to

be more flexible than zoning in requiring substantial improvement of building

sites. Third, the regulations, if properly authorized in the enabling

legislation, might be used to require reservation or dedication as open

spaces of certain high risk disaster-prone areas.

The provisions of subdivision regulations for disaster-prone areas may

typically require (1) that the location of all known earth faults show on the

plat; (2) that the location of various other geologic hazards be shown on the

plat; (3) that floodways be shown on the plat; (4) prohibition of encroachment

upon active faults or floodway areas; (5) the placement of streets and public

utilities so as to minimize the effects of disasters.

A typical provision in a subdivision ordinance may require that no plat

will be approved for development in areas subject to probable severe disasters

unless reasonable provisions or improvements are made. In this regard, subdivision regulations commonly authorize the attachment of conditions to the

approval of any plat. These conditions must reasonably promote the general

wel fare

.

C . Building Codes

Building codes are regulations enacted under police powers to guide the

design and construction of buildings or structures. The codes attempt to

protect occupants, neighbors, and passers-by from structures which may

jeopardize their health, safety or welfare.

The content and scope of building codes vary widely from state to state

and municipality to municipality. In some areas, codes deal not only with

the physical framework of the buildings, but also with plumbing, electricity,

elevators, and other mechanical functions. In addition, codes can be con- cerned not only with new buildings, but also the repair and/or modification

of existing structures. Standards for building occupancy may also be included

in building codes.

Building codes generally set minimum standards for construction methods

and materials. They do not regulate the location of developments, but are concerned with how development is carried out. Building codes are one of the

most significant means to reduce the effects of natural disasters when they

include performance specifications which: (1) restrict use of materials

which are vulnerable to the type of disaster common to the area; (2) require

structural design consistent with the expected ground shocks or flood velocities; (3) require consideration of the concept of risk as a key element in

the selection of design criteria; (4) require the performance of special

engineering or geologic studies of the site based upon the type of hazard

exposure (e.g., seismic refraction surveys to determine fault locations).

145
Building codes must be reasonably based on the principles of general

welfare. The codes must provide standards to guide administrators in issuing

or refusing building permits. Ordinances may take either a detailed specifications approach or a performance standards approach. Rather than restict

construction to narrowly defined methods and materials, performance standards

define a performance objective and allow the builder flexibility in his choice

of materials and methods to meet the objective. (Performance standards may

also be found in many zoning ordinances.)

Building codes therefore appear to have considerable potential in reducing disaster losses. Courts appear to have been generally receptive to these

regulations. However, building codes appear to have several limitations.

First, they cannot be used alone to control land use, but must be used in

conjunction with zoning or encroachment. Second, building codes which include

disaster mitigation requirements will require more detailed information on the

specific hazards to be addressed; information which is often lacking.

D . Land Use Ordinances and Building Codes Interaction

An understanding of the nature of the hazard problem is necessary to

comprehend the social, technical, administrative, political, legal and

economic issues in regulation of disaster-prone areas. Only with this

complete understanding can regulations play an important and effective role

in reducing disaster losses.

Within the general area of regulations most governmental units are faced

with a serious problem of coordination, uniformity and changes in policy

making, thus making the role of land use planning in disaster mitigation a difficult one to define. Some of the problems in this area are supported by

a study by the University of California, Irvine, entitled "Building Housing

Zoning Codes and Their Enforcement" which concluded that within the County of

Orange, California and its incorporated cities there exists:

1. "A lack of uniformity in zoning codes and their

enforcement, which leads to confusion, misunderstanding, and quite often costly operations among

governmental agencies, builders and developers.

2. "A relatively uniform set of building codes, but

a non-uniform method of code interpretation, which

tends to reduce the value of code uniformity." [9]

Special provisions within the administrative framework of existing

zoning, subdivision regulations, and building code programs should be made

for the unique features of any hazard provisions in the form of more comprehensive and interrelated ordinances. Provision for seismologic, geologic

and engineering expertise seems essential to determine earthquake hazards,

locate faults, evaluate the effects of specific uses, and impose specific

conditions for development. Coordination of the various government structures

is vitally important in order to effectively use the existing agencies and

link enabling legislation with authority for enforcement.

There does not now appear to be much planned interaction between the

land use ordinances and building codes. Further, under municipal corporations

law there seems to be a distinction between building codes and zoning. Specifically, a building code applies to the construction, maintenance and repair

of buildings; whereas hazard zoning may restrict methods of construction and

repair of buildings, but only in accordance with the community general or comprehensive plan pertaining to land use. Further, building codes generally apply to the whole governmental area,

and there is no distinction between districts as there is in the zoning

146
ordinance. This is not to say, however, that building codes could not be

developed for districts, but only to point out that, to the best of the

authors' knowledge, no such regulations have been adopted.

It is in this area that the general state of the art needs to be upgraded.

It is possible to improve our codes by applying already existing knowledge in

a manner which all parties involved can comprehend. To make no provisions in

the codes, because we cannot accurately predict a disastrous event, is to make

no decision and to reject the logic of probabilities and the knowledge gained

from science.

Codes cannot be perfect and accurate for all situations. Thus, they

must: (1) reflect the concept of risk and uncertainty; (2) be dynamic in

allowing for amendment resulting from new knowledge and improved understanding;

(3) be rationally interrelated and tied to a plan which considers probable

forms of natural disasters among its elements; (4) be based on a logic which

the legislator, administrator, and citizen can fully comprehend; thus, allowing for effective participation in the decision-making process.

III. Planning Responsibilities in Disaster Mitigation

"Planning, it is said, leads to the formulation of plans; implementation

is concerned with carrying them out. The intervening critical step is a decision." [10]

PLANNING LEADERSHIP IN GOVERNMENTAL ORGANIZATIONS

In any discussion of planning, the location of the decision making

function within governmental structures becomes a matter of concern. It is

easy to arrive at the conclusion that the most efficient way to plan -- both

in arriving at decisions regarding final and acceptable plans and the techniques for implementation -- is within a governmental structure in which

decision-making is highly centralized. Such an arrangement makes it fairly

certain that any plans developed will be accepted and will be implemented as developed. However, in a system subject to the changing views of a single

leader, or in the event of a complete change of leadership, the understanding

of the plans developed and the continuity of implementation will probably be

totally interrupted. Thus, a long-range planning process will be difficult

if not impossible to develop.

In almost total contrast to the potential effectiveness of planning

within a centralized leadership system, is the extreme difficulty of being

able to develop acceptable plans in a governmental structure where there is

extremely diffused leadership. This may occur in many situations as a result

of the current phenomenon of citizen or community participation.

Since planning activities must often respond quickly and efficiently

to issues and problems that arise unexpectedly, the diffused leadership or decision making mechanism appears not to be as effective a method of dealing

with hazard or disaster situations as might be desired.

Recognizing the problems which exist between the extremely centralized

planning function and that which is decentralized and diffused, it becomes

necessary to look for a focal point for planning in a democratic governmental

organization. The location of that function within local government has

occasioned a debate within the planning profession that is almost as old as the profession itself. The earliest concept of city planning in this country

held that it should be an independent activity within government, apart and

aloof from the bias and influence of politics. The city planner served the

selected citizens of the planning commission as a "technical" expert. The

147
planning commission was in turn, advisory to the city council. The resulting

ineffectiveness of the planning function under these conditions was inevitable,

and was brought to general notice in a classic text. The Planning Function in

Urban Government, Written by Robert A. Walker [11].

Since that time it has been argued that the professional planner and his

staff should serve directly under the chief executive, the position Walker

favored. Others, most notably T. J. Kent, Jr.; believe that the "city planning process should be designed to involve directly and continuously the city

council and the city planning commission, and not just the city planning

staff and the chief executive." [12]

The best location for the planning function in a democratic governmental

organization would seem to be that which is directly responsible to the chief

executive function. Since, in an organization of this type, the executive is

ostensibly responsible and responsive to the legislative structure, the

planning function thereby becomes directly related to that legislative activity. It may sound trite to repeat the phrase that one should not have responsibility without authority, but an executive operation cannot make effective

use of a planning function unless that executive also has the authority to

implement the plans which have been made. This must take place within the

overall budgetary and policy direction of the legislative organization.

It is often said that the executive is, in fact, the responsible planning

function within a governmental structure. In any organization of any size,

however, the executive will need professional and technical support in order

to fulfill that planning function. If that function is too decentralized from

the executive activity, it may suffer and lose its effectiveness. Experience

at all levels of government has shown often with tragic results, that if the

general planning function is placed into sub-departmental status under functional line departmental activities , there will never be a comprehensive

nature to that planning activity

.

Range of Planning Functions in Government

A . General Physical Planning

Planning functions can range very widely. Some are so limited that most

professionals would not even call them planning. They are simply a response

to the pressures that are placed on governmental agencies, (e.g., responding

to development pressures with subdivision regulation and zoning ordinance

variances, conditional use and appeals).

B . General and Functional Physical Planning and Planning Administration

From these specific regulations in ascending order of importance, the

next level of the planning function is the development of general planning

for physical development within the appropriate geographic area. This

includes the preparation of reasonably long-term projections and plans for

the area covered as well as the necessary implementation and administrative

mechanisms.

At this level, a large number of planning activities remain within the

operational departments of each government organization. The degree of

sophistication and competence of planning in these departments in relation

to the central planning function will depend on the authority of the governmental unit considered and its understanding of the general planning process.

Also important is a willingness to subordinate objectives to the higher level

goals and objectives of general planning.

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c. Comprehensive-General Planning

Next is the beginning of a comprehensive planning function, which would

pull together in one agency the various activities in the next level of

sophistication beyond the physical development process. These will deal with

the physical environment, the social environment, and the economic structure

of the community being planned.

D . Comprehensive-General and Functional Planning

This comprehensive level can be further structured and refined when all

of the various aspects of comprehensive planning are brought to bear, not

only at the comprehensive level, but also at the functional planning level.

This may be best effected through a restructuring of the organization into

one comprehensive planning organization.

E . Total Planning - Including Budget Preparation

American governmental opinion may be ready for a rationalization of

planning organizations which would pull together all the various comprehensive

and also functional activities. As Chapin has said, "there has been a growing awareness of the importance of integrating planning more directly with policy

formulation in the political process." [13]

Such a comprehensive planning organization could plan budgets in a form

appropriate for adoption by the appropriate legislative body. Specific items

can be designed and detailed to the point where the legislative body can evaluate the proposals in light of the policy implications, and finally

approve the general policy and the budget for implementation.

All activites which take place prior to budgeting would be handled by

the comprehensive planning organization whereas all following activities

would be handled by various levels of the organization concerned primarily

with implementation. This means that the planning agency develops the general

plan to be followed by more specific sub-area plans. Within and subservient

to these, would be developed specific locational plans for parks, schools,

sewage facilities, libraries and circulation facilities.

Disaster Mitigation Within the Range of Planning Functions

Reviewing this range of planning functions, it would appear that land

use planning for disaster mitigation would be easiest to apply at the lower

levels. For example, planning for hazards could occur first as an element of

the Comprehensive-General Plan (as is now required in the case of earthquakes

by the State of California). It would result in actions in other departments

such as Building and Safety, thus, implementation becoming dependent on the

individual agency evaluations of the need and feasibility for such planning.

Planning for natural hazards at the lowest level, would then be administered

through regulations appearing in codes and ordinances.

Such an approach to inclusion of hazard considerations within land use planning could result in fractionated, uneven and ineffective administration

of whatever programs emerge. It should be apparent from previous discussion

that whatever policies are used to address the problems of hazards by government, they must have been considered and agreed upon at the highest, most

comprehensive level of planning and policy formulation. Only at the highest

level can concepts such as risk be viewed within a framework capable of

integrating planning for hazards with the social, economic and physical com- ponents of the community. Without this integration, a rational approach to

the decisions needed to formulate politically feasible hazard mitigation

149
standards cannot be made.

At the highest level, however, an evaluative mechanism must be established as part of the over-all planning process to enable a balanced analysis of

various alternatives, i.e., location, risk, cost and timing. On that basis,

recommendations can be prepared leading to adoption of standards and allocation of responsibilities required for incorporation of hazard mitigation

planning into land use planning. This will enable the legislative body, in

light of the comprehensive and balanced presentation made, to rationally

determine the allocation of various resources and costs and to assign responsibilities for implementation of the approved policies and programs to the

appropriate agencies.

Such a planning approach would make it possible for the executive or

the legislative body to analyze the multitude of options, within the acceptable levels of risk, the acceptable levels of costs and restrictions, and

the generally limited resources available. It would also permit the balancing

of these elements within a policy framework recognizing political implications.

At that point, the various political implications could be placed within a context of a rational evaluation mechanism. If those recommendations were to

be overruled, it could be done on the basis of some rational and factual information, rather than on the basis of intuition or emotion. Also, a rational

risk mechanism could be built into the evaluation of the various alternatives,

so that legislative or executive decision-makers would have a rational and

comprehensible mechanism within which to make and to support their risk

deci s ions.

There is, however, a risk in this system which should be pointed up.

That is, due to the difficulty in comprehending the various ramifications and

interrelationships of the total process, the legislative or executive function

may abdicate its decision making authority to the various professional administrators operating at lower levels of responsibility. The result of this

situation is the bypassing of the political process; allowing the opportunity

for the professional administrators to make political decisions.

An effective legislative or executive function can, and must, take the

responsibility to make comprehensive decisions which relate various costs,

resources, needs and constraints to each other. This can be done providing

a comprehensible and comprehensive plan is used as the basis for judgments.

Delegation of Planning Function to Governmental Levels

Definable levels of government can be identified as follows: international, national, inter-state region, state, intra-state region, county, city,

district and neighborhood.

It is obvious that planning takes place in varying degrees at all levels

within governmental structures. It is equally obvious that almost everyone

would agree that different governmental levels have varying and, hopefully,

supplementary functions. Most analyses of regional decision making have run into difficulty when specific assignments had to be made as to the responsibilities of specific agencies. Because of the complication of these intergovernmental relationships, a set of criteria has been developed to aid in

determining the responsible planning level [14]. A set of ten planningrelated actions were specified which, when implemented as a system, make up

the total process. These ten actions are:

1. Inventory - survey of existing conditions, e.g., physical, social,

and economic.

2. Analysis - review of implications and interpretations, based on the data developed in 1.

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3. Policy setting - formulation of policy priorities, strategies,

specific areas of concern, etc.

4. Plan making - applying the results coming from 1, 2 and 3 - to

the specific locational or functional area of study.

5. Standard setting - devising specific criteria, levels of conformity

and physical standards.

6. Acquisition - where necessary, make provisions for funds to accomplish ends, noting the sources of the funds.

7. Regulation - application of public methods, techniques, ordinances

and laws, to achieve policy.

8. Development - improvement or construction of facility.

9. Monitoring - ensuring that standards are maintained or modified as experience indicates.

10. Coordination - ensuring that all elements of the programs are pursued in an organized and unified manner and at the appropriate

level by various agencies.

The following is a series of criteria by which each of the above actions

can be measured in order to find the appropriate governmental level to which

responsibility should be assigned.

1. Responsibilities for inventory must be undertaken by the levels of

government that will: be the units that will utilize the data; have

within their jurisdiction the information to be surveyed or inventoried and have data accessible or available to it; have familiarity

with the data to be collected; have the facilities and manpower to

collect the data, and collect data efficiently.

2. The levels of government that undertake responsibilities for

analysis must: be technically equipped to handle data (including

manpower and machinery, and the possibility of computerization,

data-banking, long-term storage, and retrieval); serve as a clearinghouse for all similar data; have expertise and competence

in dealing with such data; understand all aspects of the problem

so that analysis reflects both explicit and implicit needs; provide

the greatest efficiency in conducting the analysis.

3. The levels of government that undertake the responsibility for

policy-setting must: have the clearest perspective on the issue;

have some potential for implementing policies; have access to basic

data and analyses; be accessible to the public; be free of conflicts

of interest; be sufficiently flexible to alter policies quickly, as needed; and be able to relate proposed policy and its long-term

impact

.

4. Plan-making must be the responsibility of the governmental level

that is: able to define the plan to make it clearly correspond to

basic policies but remain workable; has a technically competent

staff to develop the plan within a reasonable time; has familiarity

with the subject of the plan being developed; and has access to

basic data collected.

5. The setting of standards must be undertaken by the level of government that will: include inputs from units affected, or in some way,

impacted by the standards being developed; represent no special

interest and be free of conf 1 i cts-of-i nterest ; have access to the

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specialized skills and equipment necessary for detailed standardsetting, and have flexibility to respond to new information and data

to change standards rapidly to reflect that information.

6. The levels of government that undertake the responsibility for

acquisition must; be the one having the financial capability and revenue base; be able to achieve economies of scale in the funding

of the activity; have the legal authority or options to finance

such activities (e.g., assessment districts); be responding to its own policies in regard to the population being served.

7. The government level responsible for regulation must: be the one that is able to deal with the problem most directly; geographically

covers the entire areas for which regulation is necessary; be able

to provide optimum coverage of the issue (not geographic, but for

the full range of issues involved); have the competence to draw up

documents and ordinances to best and appropriately describe the

action and activities desired; be directly related to the issue of

concern; be large enough to ensure efficient enforcement of the

regulation; and be accessible and controlled by the service population.

8. The governmental level that undertakes the responsibility for

development must: be geographically and technically familiar with

the project; be one that ultimately has a role in the ongoing

operations functions of the development; have some element of the

project occur within its jurisdiction; be able to ensure efficiency

and effectiveness in development; and be able to alter or affect

existing regulations or sub-policies to satisfy or accommodate

requirements for development.

9. Monitoring must be undertaken by that level which has the authority

to deny or cease activities that do not conform with standards; be

able to maintain monitoring on an ongoing basis; be able to provide

optimum area coverage (efficiency of time, material, geography);

include monitoring as part of another agency so that monitoring is

aligned with policy and not just standards; and have the manpower

and expertise to conduct the monitoring operations.

10. Finally, the levels of government that undertake the responsibility

for coordination must: have resources and competence to allow for

coordination, and be capable of ensuring coordination by other

levels of government.

Based upon these criteria a "functi onal -responsi bi 1 i ty matrix" can be

developed which would identify the various levels of governmental responsibility on one axis and the activities or responsibilities which must be

assumed on the other. This would then permit the rational matching of the

responsibility and function with the most appropriate governmental agency or level in practice. This is most difficult, however, due to the fact that

responsibility is very diffused between governmental levels.

Regional Government and Planning for Disaster Mitigation

Land use planning has, by tradition, been almost totally a matter of

local concern. There is ample evidence gathering, however, that higher

levels of government will be increasingly involved in these matters.

President Nixon has cited the need for national land use planning

policies [15] and several bills are pending related to this need, but no specific action has yet resulted.

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state level assumption of land use planning responsibility and authority

is, also, increasingly common. "State governments see themselves as strategically situated to plan and manage the environment, particularly in the

matters of land use and waste management. Local governments are too close

to the economic and political pressures that create environmental problems

to be sufficiently detached", [16] is a common viewpoint in the current trend

Land use planning related to natural hazards can be seen as a sector of

environmental management and thus, subject to the same considerations for

governmental controls.

In the Quiet Revolution in Land Use Control, Bosselman and Callies

describe a number of planning programs in which states as well as interstate

and intrastate regional commissions have successfully addressed environmental

problems which could not have been encompassed by local jurisdictions. "The

innovations wrought by the 'quiet revolution' are not, by and large, the

results of battles between local governments and states from which the states

eventually emerge victorious", [17] but are instead a cooperative venture in

intergovernmental coordi nation.

This is the large view. Examined more closely, local governments, like

any other organization, do not welcome incursions into their realms of

authority or a continuing erosion of their powers.

Local planning agencies can often show that the reason they have not

been effective in solving environmental problems is that the state has not

granted them the necessary authority. Local land use plans often note

natural hazards, but the implementation powers necessary to correlate land

use regulation to hazard are lacking so no such provisions can appear. For

instance, the Los Angeles City Planning Departments' preliminary General

Plan for the Santa Monica Mountain Area, written in 1962, shows a full treatment of earthquake fault lines, geologically unstable areas, fire prone areas steep slopes, and other hazards to development. Yet, in spite of these

admonitions, pressures for permitting greater residential densities and more intensive land uses have been unremitting.

California's approach to new environmental planning concerns, which will

probably include land use planning related to natural hazards, has been

two-pronged. One trend has been toward planning and management at the

regional scale. The Water Quality Control Boards and the San Francisco Bay

Conservation and Development Commission are two examples often cited as outstanding successes. Although as will be shown. Councils of Government,

such as ABAG (Association of Bay Area Governments) more often serve as a reminder that the simple act of forming a regional planning group does not

insure efficient or effective problem solution.

We believe that planning, as a means of mitigating the effects of a disastrous event should be undertaken on a regional basis.

However, the history of regional concerns, especially as related to

planning, has been beset by many difficulties. There is no question that

many people are concerned about the philosophical implications of the develop

ment of regional approaches to solutions. If, however, governmental agencies

had been able to develop satisfactory regional coordinative systems, these concerns may have been overcome. In the development of regional programs and

activities, professionals have looked for the perfect and al 1 -encompas s i n

g

solution and, not being able to find that perfect solution, have settled on extremely limited single purpose implementation mechanisms. As an example of

the confusion which results, consider the following:

"Who makes the environmental development decisions in the

San Francisco Bay Area?

"The pattern of government there includes one state government. There are, in addition, nine counties, 110 city

153

494-610 O - 73 - U
governments and 604 special districts. Of the latter, 294

are governed by elected boards or commissions, while 310

are governed by one or another of the county boards of

supervisors. In addition, there are 213 elected school

boards, thirteen appointed local housing authorities and

sixteen appointed redevelopment or renewal agencies. This

is a total of 966 decision-making bodies.

"That is not the whole story, however. The legislature

apparently has felt that the people of the Bay Area cannot

completely govern themselves this way so it has created,

or permitted to be created, several other agencies: There

is a state-created and appointed San Francisco Port

Authority, a San Francisco-Oakland Toll Bridge Authority

and, most recently created, a Bay Area Hospital Planning

Committee. Also, the San Francisco Bay Conservation and

Development Commission is charged with licensing permits

to fill the shores of San Francisco Bay while, at the same time, it is required to prepare a comprehensive plan for

the bay's development. There is more: In its wisdom, the

1965 legislature directed the state Water Quality Control

Board to prepare a comprehensive plan for disposing of all

liquid wastes in the entire drainage basin of the Central

Valley and the San Francisco Bay.

"On top of this, there is the legislatively created Bay

Area Transportation Study, which is to plan comprehensively,

in accordance with the Highway Act of 1962, for highways

(not really transportation) in the Bay Area.

"Sitting above and aloft is the Association of Bay Area

Governments (ABAG) organized under the Joint Exercise of

Powers Act. Its charter gives it the responsibility for

comprehensive planning for the area. Although ABAG has

taken on the overall planning job, it is really a voluntary

debating society from which any of its members may

resign at any time. It acts, on the one hand, like the

United Nations, a collection of sovereign states. On

the other hand, when there is a need for more aid, these

local governments run to the legislature to be bailed

out

.

"There are four super-regional special districts, that

operate their own fiefdoms, each in its own way creating

the Bay Area environment without reference to anyone or anything else. The East Bay Municipal Utilities District

lays the water and sewer lines over most of the eastern

part of the bay. It is governed by an elected board.

And I will guarantee that 99 per cent of the electorate

in the communities which it serves do not know the name

of one member of that board.

"The Golden Gate Bridge and Highway District is a device

created to build and operate one bridge: the Golden

Gate. It is not part of the Toll Bridge Authority.

"The Bay Area Rapid Transit District is going to spend

close to a billion dollars on transit. It is separate,

too, and not related to highways or regional transportation planning.

"Finally, there is the Regional Air Pollution Control

District. This group, made up of locally elected

municipal and county officials, has done a fine job.

154
which gives one hope that broader aspects of the police

power can be exercised on a regional basis through this same kind of politically representative and responsible

body

.

"All in all, it is estimated that there are over 4,500

officials, 3,500 of them elected, who make the decisions

that affect the development of the environment in the

San Francisco Bay Area." [18]

A region should be dealing with such issues as general planning, transportation, air and water pollution, air and waste generally, the supply and

dissipation of both, disaster mitigation, and many other non-physical concerns

such as health services, law enforcement services, etc. In the past, as we have seen, it has been the practice to establish specific regional combinations

of cities and counties for each of the particular functional needs. In only a very few situations have the boundaries of various regional agencies been

coterminus. Because of the variety of these special designations, it has been

difficult to interrelate the various planning activities being undertaken. It

is therefore, almost impossible to establish any kind of a comprehensive planning framework within which all these various activities are undertaken.

An additional major difficulty concerning a rational alignment of regional

activities lies in the fact that local governments in the undertaking of various functional activities are depending very heavily on varying and often very

confusing funding sources from both the federal and state levels. These funding agencies also require various regional review bodies, each with varying

government bodies totally unrelated to each other.

It is strongly recommended that a general and probably national policy

be adopted and that a set of regional alignments be established. These should

be applicable for all activities which require a regional concern. Even

though these regional alignments may not be perfectly suited to any of the

specific functional requirements, they should be designed in such a manner

that they will meet a substantial number of the most common overlapping

concerns. All state and all federal agencies with their funding or other

assistance programs should be required to work through the same regional

alignments, and thus the current confusion, contradiction and competition - all of which tends to produce nothing but a questionable use of public resources - may be avoided.

Such a solution will not satisfy the administrative perfectionist, but

should allow for a more effective management of public resources.

With specific reference to disaster mitigation, a public agency should

be able to respond immediately, and should not be forced to screen a handbook

of resource organizations to find the appropriate regional, state or federal

agencies to be addressed for immediate assistance.

Considering the possibilities of this approach to environmental management, (and particularly land use planning related to natural hazards) a more effective and efficient model for governmental administration must be formed.

Broad statutes enabling implementation powers for land use standards related

to natural hazards must be provided by the states. Objectives for regional

and local planning related to hazards would be defined in the parameters of

authority set forth. Regional commissions, responsible to the state, would

analyze hazard impacts and degrees of local area vulnerability to specific

natural phenomena. Data, guidance, and technical assistance would be offered

to local agencies in formulating and implementing General Plan elements by

the regional governments. Local plans, codes and policies would be reviewed

by regional agencies and minimal standards enforced, beyond which local

option would prevail. Coordination of local policies would be achieved at

the regional level, and regional coordination at the state level. Appeals

155
from local action would follow the same hierarchy. Data, policies and programs would be scaled to the area needs of the three levels.

True comprehensiveness could only be achieved in this model if the

regions were held constant, that is, occupied the same areas for all functions

rather than being separately delineated according to the problems addressed.

If this were true, land use planning related to hazards could be rationally

coordinated with all land use planning policies and related by consistent

programs of social and economic concern.

This is the bare bones of a rational model. Political feasibility,

however, is rarely measured in terms of rationality. The resistance of local

governments to further administrative constraints, the resistance of the

public to acceptance of the costs of a third level of government, the resistance of private enterprise to another level of police, permit, and license

authority, are but a few of the obstacles in the path to the rational model.

IV. Natural Disasters That Can Be Mitigated Through Land

Use Planning

Earthquake

Seismic safety through land use regulation implies a consideration of

the various hazards associated with tectonic earthquakes in terms of density

and location of new structures and facilities, the rationale and criteria

utilized in site and structural design, an evaluation of existing structures

and facilities, and a review of the efficacy of existing codes and ordinances.

These criteria must be based upon a consideration of the various modes of

failure that can occur during an earthquake in both open and developed areas.

A . Ground Shaking

The majority of damage to man made works from earthquake is caused by

earth vibration as differentiated from localized damage due to permanent

ground movements, water forces and flooding. However, there have been cases

of significant losses as a result of the latter which will be discussed

1 ater

.

Structures can be designed to survive severe earth vibration with little

damage but it is not usually considered economically feasible to design

structures to withstand permanent ground movements or tsunamis. Damage due

to earth vibration is a function of the overall earthquake resistance of the

structure and the intensity and duration of ground vibration.

In a severe and prolonged earthquake, the area of intense ground shaking

(MM IX and above) will likely cover an area in excess of 1000 square miles.

Within this area, there will be severe structural damage to modern buildings.

Variations in damage within this same area can be traced to differences in

intensity of surface vibrations as affected by soil conditions, epicentral

distance, azimuth from source to site as well as direction and extent of

faulting (energy release). Also the soil structure interaction will cause differences in response of various types of structures. The variations must

be provided for in building codes and zoning regulations. Codes and regulations, can be operationally improved by zoning risk maps which (a) show the

earthquake response characteristics of particular sites within mapped area,

and (b) require additional in-depth investigations of sites located in mapped

areas, and/or vulnerability of existing structures located on the sites.

The severity of shaking that can be reasonably expected within the

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lifetime of a structure is a function of the seismicity of the area on which

it is located as well as the areas relationship to major fault locations and

directional trends. In Southern California, for example, the equivalent

statistical risks for shaking (excluding site conditions effects, based on historical data) vary by as much as 50 percent within a distance of forty

miles. The risk from this parameter can also be treated by zoning regulations

which are interfaced with building codes.

On a regional basis, the Uniform Building Code (the principal U . S. Code

that treats earthquake) utilizes an earthquake risk map which shows large

zones of equal seismic probability. The code forces for aseismic design vary

for the dii^ferent zones as follows:

Zone Portion of Force

0 None

1 1/4

2 1/2

3 1 Present U. S. Codes do not provide for differences in soi 1 -structure

interaction. In order to adopt this factor, it would be necessary to have

much more information including the dynamic response characteristics of soils

and seismic velocity profiles of adjacent areas. Presumably, if this information were available, risk maps could be made to guide local agencies in the

foundation of codes which would include soi 1 -structure interaction as a function of seismic safety for structures.

A code which recognized these variables would be complex and very few

local building departments, even in Southern California, much less in the

midwestern and Eastern parts of the U.S., are currently manned to administer

such a code.

All facilities need not be designed to the same level of safety. Important and critical structures such as those housing medical, emergency and

utility services should be required to have a higher level of safety, or accept a lower level of risk, than ordinary commercial and industrial building. Thus, different interpretations of risks maps should be provided in the

codes for the design and location of these structures.

B . Permanent Ground Movements

Earthquake damage to man made structures caused by permanent ground

movements (landslides, lurching, liquefaction and fault break) can be effectively treated by land use planning. In order to accomplish this, it is

necessary to define and locate potentially hazardous areas, rate them as to

risk relative to ground shaking, and then reduce the additional hazard of

permanent ground movements by proper engineering. Prohibition of certain

types of construction based on preassigned risk allowances, may be necessary

in some areas, subject to some of the hazards considered below where engineering is unfeasible.

B.l Landslides and Lurching

Scientifically prepared and properly administered grading codes and land

use regulations have essentially prevented landslides in the City of Los

Angeles. There are no known landslides on sites prepared after 1963 when the

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current grading code was adopted in the City of Los Angeles. Published data

[19] clearly indicates that scientifically prepared and properly administered

grading codes are effective in preventing loss of life and property. It is

presumed that these or upgraded codes will mitigate the effects of earthquake

caused landslides.

Chapter 70 of the Uniform Building Code is a minimal code which, if

properly administered, can be extremely effective. It requires that

engineering measures be applied in certain areas to control landslides and

settlement. In addition, code provisions cover fill compaction and foundation design requirements which are intended to minimize damage due to

sett! ement

.

However, even with these regulations, there were many instances of

settlement and landslide damage in the San Fernando earthquake of February 9,

1971. Fills settled along freeways and roads, landslides and settlement

occurred in the vicinity of the Jensen Filtration plant, Sylmar Converter

Station and L. A. County Juvenile Facility [20].

It must be borne in mind that codes alone are of little value, unless

there are adequately educated and trained personnel available to staff the

enforcing agencies. This staff must include qualified and possibly licensed

personnel in the following disciplines: (1) Architecture or Land Planning,

(2) Engineering Geology, (3) Soil Engineering, and (4) Civil Engineering.

Above all, quality control is an essential ingredient.

B.2 Liquefaction

In a major earthquake which affects coastal areas, there are usually

severe problems associated with saturated natural and man made fills. A phenomenon known as liquefaction occurs where the earth vibration causes saturated silty-sandy soils to subside rapidly and flow laterally.

Land use control measures for this failure mode must (a) limit the use of the subject area for the construction of certain critical facilities, (b)

propose better engineering design practices for structures and soils and/or,

(c) accept certain risks for special facilities (like harbor facilities)

which could be damaged severely, but which also must function daily for the

economic survival of an area.

B.3 Faulting

Damage as a result of surface fault rupture has been common in California

earthquakes. When fault ruptures extend to the surface and through man made

structures, severe damage usually results. It is generally not considered

economically feasible to design structures to resist these movements. Therefore, potentially dangerous faults should be located and avoided, if the risk

level warrants such action.

It is doubtful that the present "state of the art" will enable us to

accurately locate all of the potentially dangerous areas, except on a very

broad basis. For instance, the San Fernando-Syl mar area in 1971, the White

Wolf fault in 1952, or the 1940 El Centro fault were not recognized as potentially dangerous areas from the standpoint of surface fault rupture.

It is possible to map the more well-known faults such as the San Andreas,

San Jacinto, Hayward and other fault systems. However, the task of mapping

potentially hazardous surface areas for the majority of other faults throughout the country would be tremendous.

The problem of mapping is complicated by the fact that some active faults

such as the Newport- I ng 1 ewood are covered by several thousand feet of alluvium

and even if a surface break occurred, due to movement on this fault, it would

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be very difficult to predict where it would intersect the surface. Then the

only way to positively eliminate the hazard would be to prohibit development

in a wide area on the surface over the fault zone. Such a plan would probably

not be economically sound. Perhaps it would be better to accept the risk in cases like this provided a quantitative risk of Fault Activity Relative to

Ground Shaking Risk can reasonably be identified.

Fault breakage is critical in the case of public utility systems, such

as water, gas, and electricity. In this area, systems planning with respect

to potential surface fault breakage is essential. Where these "City Lifelines"

cross active faults there must be provisions for prevention or rapid repair of

damage

.

Public utility systems must be planned so that, if portions are damaged

at active fault crossings, there will be alternative systems and storage

resources available for immediate restoration of service. There has been

some limited activity in systems planning for catastrophe but there is a real

need for a more significant effort in contingency planning in this area.

The problem of land use planning in relation to active fault locations

in an area such as California is very difficult. There are so many potentially

active faults tliat it is impossible to prohibit construction in these areas. Appropriate fault activity risk, tied with planning and design restrictions,

must be accepted or tolerated. In other areas of the U. S., fault mapping is

virtually nonexistent. Unless this mapping is accomplished, land use planning

relative to active faults would be meaningless.

C . Water Forces and Flooding

We can see how various modes of failure are interrelated (e.g., a landslide can trigger a flood by causing a dam to overflow a breech).

C.l Tsunami

A tsunami is a rare type of sea wave that is produced by (a) large

submarine, near or distant, earthquakes, (b) earthquakes whose epicenters are near a coastal area, (c) large submarine landslides, or (d) volcanic eruption.

The first recorded Tsunami dates back to 1400 BC, when it destroyed the

town of Amnisos, Crete. Subsequently, the Greek town of Helice perished

under the effects of a tsunami. More recent examples of other damage due to

tsunamis are:

April 1, 1946: Hawaiian Islands - damage on the five

main islands; listed as Hawaii's worst natural disaster

with 159 dead and an estimated loss of $25,000,000.

May 22, 1960: Chile - a violent earthquake in Chile

propagated tsunami waves that traveled across the Pacific

Ocean killing people as far away as Japan. The loss in

Chile alone, was 4,000 dead and an estimated loss of

$400 ,000 ,000

.

March 28, 1964: Alaska - "Good Friday" earthquake

generated tsunami waves that destroyed much of Valdez,

Alaska and extended death and destruction as far away

as Crescent City, California.

In 1948, two years after the 1946 Hawaiian tsunami loss, the "Seismic

Sea-wave Warning System (SSWWS) was initiated by the U.S. Coast and Geodetic

Survey. The first major tsunami after initiation was in 1952, with a giant

tsunami generated near Kamchatka, Alaska. The waves caused $800,000 of

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damage, but no loss of life. Since then, the system has been expanded to

most of the Pacific nations. The Chilean earthquake-induced tsunami in 1960 was the most destructive since the initiation oi" the SSWWS system. The wave

left thousands dead in Chile. Hundreds and possibly thousands of lives were saved by the SSWWS system in other areas. Another precaution can be found in the telephone directory for the Island

of Hawaii. Five pages are devoted to explaining the tsunami warning system

(SSWWS), showing graphically the areas that may be inundated and must be

evacuated. Ilany of the newer buildings are designed to be tsunami resistant.

For example, one hotel in Hilo and one on Oahu are designed with essentially

breakaway walls at ground floor level in combination with tsunami resistant

columns to allow the waves to pass through the lower floors. Based on present

knowledge, it should be possible to identify and map potentially hazardous

areas considering the threat of tsunami. These hazardous areas would undoubtedly include most of the existing heavily industrialized and commercialized

waterfront areas. The imposition of building restrictions on existing structures would be expensive and difficult. The tsunami hazard must however be

considered for new construction and for very important structures such as nuclear and fossil fuel generating stations.

C.2 Flood Due to Dam Failures

There has never been a failure in the U.S. of a modern compacted earth

fill or concrete dam due to earthquake. Those earth dams which have suffered

damage, were the older dams constructed with hydraul i cal ly placed fill. One

obvious way to eliminate the hazard of earthquake induced dam failure would

therefore be to identify, remove or strengthen all old hazardous dams. Such

a program is underway in California.

Land use planning would be effective in reducing the hazard by evacuating,

or prohibiting building in areas below hazardous dams. However, it would seem

that the preferable solution would be to remove or correct the hazardous dams.

Wind

The hazard of damage and loss of life due to strong winds has some parallels to the earthquake hazard. Both concern lateral and uplift loadings

for which structures are not normally designed. Experience has shown that

structures which are especially designed to be earthquake resistant are also

highly wind resistant. Both hazards have been evaluated on a regional basis

by providing "risk" or loading maps in building codes.

The Uniform Building Code provides a map of the U.S. which shows

resistant wind pressures and locations of localized winds such as Santa Ana,

Chinook, Columbia River Gorge and Wasatch Mountains. The choice of code wind

loadings is largely left up to local jurisdictions. Frequently, local choices

do not agree with wind loading maps, nor do they recognize localized wind

condi ti ons .

A . Hurri canes

One of the major wind hazards is the tropical hurricane which is generated in the low latitudes (8°-15°) and often travels northerly into the

southern states (some from the Pacific Ocean may extend infrequently into

Southern California). The average annual loss from hurricanes is estimated

to be $440,000,000.

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B . Regional Winds

Regional wind conditions are created by cold, strong, regional winds

passing over mountain ranges, descending on the lee side, and encountering

a warm low pressure area with subsequent high velocity winds. These winds

are often referred to as Chinook, or Santa Ana winds. They may reach

velocities up to 90 miles per hour and can create havoc. They must be con- sidered in land use planning.

The Santa Ana wind, typical to Southern California, is very strong at

the mouths of certain major canyons. Improperly placed and/or designed

structures in these areas can be subject to wind damage. Planned low

intensity use or such areas, placement of wind breaks or proper structural

and site design can compensate for this natural hazard.

The Chinook winds of the regions of Colorado and Wyoming are similar,

but often of broader scale and definitely should affect land use planning.

This is particularly true when considering the location of schools, air

fields, highway overpasses and recreational lakes.

C . Tornado

Tornado winds are of extremely high velocity and low pressures. It has

not been considered feasible to design structures to survive these winds.

Damage due to tornadoes is severe over a relatively small area and general

areas of occurrence can be predicted, but specific spots where they will

strike cannot be predicted.

D . Storm Surge

Damage related to storm surge may occur wherever strong winds can affect

large expanses of water. This includes portions of the eastern seaboard which

are subject to hurricane winds or strong surge activity related to storms in

the North Atlantic, the Gulf Coast which is battered by the greatest number

of hurricanes, the West Coast which is subject to local strong wind and surf

action, and the Northern California-Oregon coastal area which receives the

full force of the prevailing westerly winds and their wind-generated waves

and surf.

Examples can be listed where valuable property has been severely damaged

by storm surges. In recent years the Ventura County Coastline of Southern

California has undergone serious erosion and loss of valuable coastal lands

until groins (sea walls perpendicular to the coastline) were constructed to

retard the rate of erosion. The loss of sand from surge action along a few

miles of Ventura Beach averaged 90,000 cubic yards prior to the remedial

measures, for example. In addition to the loss of sand, there was an accom- panying loss of structures and an impairment of highway facilities.

It should also be pointed out that in our complex society and environment,

there are other factors contributing to storm surge erosion. These other

factors are:

a) Drought, which reduces the production of sediment through erosion

and the accompanying transporting of sand from the hinterland to

the beach.

b) Construction of flood control facilities which tend to reduce

erosion and transport power of the streams and allow settlement

of sediment in reservoirs.

There is a need for awareness of the effects of wave erosion to beaches

and wave-cut cliffs, especially in areas where population expansion has

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increased both land use and land values. New concepts are currently being

planned in these coastal zones subject to strong storm action. These include:

offshore roadways or causeways; offshore oil drilling platforms; offshore

nuclear power plant sites and possibly floating sites; below sea-level aqueducts and other similar facilities.

With regard to land use planning, one recommendation of ASCE states,

"In special locations such as on prom on tar ies, on mountains, in gorges or for other unusual reasons, where records or experience indicate that the

basic velocities are inadequate, higher basic velocities may be used at the

discretion of the engineer." [21]

Wind forces due to hurricanes are generally accounted for in the wind

pressure maps provided by ASCE. However, there is a need for more modern

wind codes which recognize the necessary variables such as height, shape

factors, gust factors, uplift forces, negative pressures and variation of

forces on different building components. The hazard of hurricanes must be

handled by engineering and building codes rather than zoning.

The existence of local high velocity winds is well known. However, the

hazard of these winds is not generally recognized by incorporation of increased

wind forces in local codes. For example, Santa Ana wind velocities in excess

of 100 miles per hour are measured every year near the Font an a- Ri ve rs i de area

in California. However, building codes in these areas do not require increased

wi nd design forces

.

Land use planning could be effective in reducing the hazard caused by

local high winds by mapping these areas and requiring higher design forces

in the high risk areas by restricting the types of structures which can be

erected

.

Volcanic Hazards

The island of Hawaii, which is the youngest of the Hawaiian Islands, has

the only active volcanoes, although Mount Rainier and a few other areas in

the United States may become active. The Hawaiian islands are built over a 1600 mile fissure or fault zone that is directly related to sea floor

spreading, faulting, and volcanic activity. Urban de vel opment ( s ubdi v i s i ons

)

is taking place currently on the Puna Coast some 20 miles Southeast of Hilo,

Hawaii. This coastal area is beautiful and possesses some of the most spectacular views that can be found on the Hawaiian Islands. However, major

flows from the Puna Rift (or fault system), which is located on the westerly

flank of the active volcano Kilauea, spread over these areas in 1955 and

1960. In fact, the majority of these new developments are located on the

1955 flows.

Technically, there is a very good possibility that the Puna Rift will

be active again - probably in the very near future. To replant papaya,

orchids, and other productive vegetation on a recent flow is one thing, but

to place housing developments in such areas is less than prudent.

The island of Hawaii is a young island, still growing, and one which

will witness much volcanic and earthquake activity .in the future. The two

active volcanoes, Kilauea and Mauna Loa, have been very active in recent and

historical times and should remain so into the future. These portions of the

island of Hawaii should be utilized for national parks ,. recreati on , and

agriculture and not for housing and/or commercial structures which will

undoubtedly be destroyed in the near future.

Other Unusual Hazards

Other hazards can result from unexpected explosions due to industrial

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activities. Both airborne pressure waves and projectiles can result from

this source of hazard.

On the other hand, quarry blasting, road blasting and other sources of

blasting set up ground vibrations which are alleged to cause damage. Only in

very rare instances have blasts of this nature actually caused any damage,

however, the nuisance has caused much concern, especially in the court rooms. Zoning could be accomplished around certain quarries or regulations

could be developed for the use of fixed charge sizes (or a combination of

both). However, zoning or regulations should be established primarily on the basis of nuisance limits, since damage from such sources is not generally

expected. If regulations are developed based on nuisance limits, court ruling

and public opinion surveys should be thoroughly documented and taken into

consideration rather than technical arguments.

Earthquakes can be triggered by changing local earth pressures such as the filling of a large reservoir (example, Koyna Dam in India and the Konya

earthquake of December 11, 1967). Removal of fluids from an oil field or repressuring an oil field may also trigger small to moderate earthquakes.

Examples of repressuring would be the quakes in the vicinity of the Rangely

Field in Colorado and the Newport-Inglewood zone in California with some quakes in 1941, 1944, and 1949 being attributed to withdrawal. During the

mid-1960's Denver, Colorado witnessed a series of earthquakes that have been

attributed by many to be directly related to the placement of fluid wastes

under pressure into relatively deep rock materials at the Rocky Mountain

Arsenal near Denver.

These latter examples generally can not be considered in zoning, but

should be regulated.

V. Current Land Use Planning Practices Applicable to Disaster

[litigation

As noted previously, there appears to have been little regard for

natural hazards in land use planning ordinances and statutes. Certain trends

are now evident, however, which may indicate imminent change in this field.

Some of these trends will be reviewed here.

Environmental Land Use Planning: A Methodology for Disaster Mitigation

Planning

The recent upsurge of interest in envi ronmental -ecol ogical problems

has introduced a spate of federal and state legislation addressed to these

concerns, accompanied by a growing number of landmark court decisions interpreting the new laws. Mainly concerned with protecting nature from man,

environmental legislation is of interest here because of its converse application, protecting man from nature.

"Man has chosen some of the worst places to live with respect to his own life and health. He has selected flood plains, aquifier recharge areas,

fault zones, unstable soils, and other equally poor environments." [22]

In order to correct man's errors, environmental planning has come into

existence. Under the Environmental Quality Act of 1969, the U.S. Government

requires that environmental impact statements be prepared for all projects

requiring federal funding support. Recommendations for regulation of land

use can be developed from these statements.

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One common methodology of environmental land use planning is well-suited

for use in hazard studies. Typically, a set of maps is developed from area data showing, individually, areas sensitive to important natural forces.

Thus, for a given region, the maps may show areas of steep slope, areas of

subsidence, locations of delicate and/or irreplaceable flora or fauna, or whatever physical systems are matters of concern to the planners. Overlaid,

the maps will show a priority of environmental protection needs within the

mapped region. For consideration of specific locations, such as development

sites, matrices are developed from the maps and the data, and used to evaluate

the site in relation to its scores on all the environmental factors to be

considered.

This same approach can be used to consider matters of natural hazards to man. In land use planning related to earthquakes, for instance, known earthquake faults, geological conditions which exacerbate earthquake damages, and

other components of earthquake damage can be shown on a seismic risk map

together with existing structures or land use patterns. The map can be

divided into grids scaled to the appropriate area of study, whether state,

regional or local. Matrices accompanying the maps can evaluate more specific

locations in relation to the degrees of hazard shown by the maps. And, as with environmental planning, recommendations for degrees of regulation

correlated with degrees of hazard can be developed from and supported by the

evidence shown on the maps.

The advantage of this methodology is that it can be presented in such a manner that it is easily understood by the layman. Thus, public understanding

and participation are possible in considering appropriate risk levels, measures

for damage prevention , density standards and regulatory provisions

.

A more detailed approach to performing an impact analysis is presented

in a document entitled, a Procedure for Evaluating Environmental Impact,

prepared as a Geological Survey Circular 645, U.S. Geological Survey [23].

An application of this methodology to seismic impact assessment is presented

in a following section of this paper.

Flood Plain Zoning: Lessons to be Learned

Flood plain zoning appears among the earliest land use planning efforts

devoted directly to improving public safety from natural hazards, and can be

considered a pioneering effort in environmental planning for hazard mitigation.

As a concept, flood plain zoning should not be confused with flood

control measures. Flood control pertains to a system of engineering devices

and structures to contain or divert flood waters; dyking, dredging, damming,

channelizing, etc. The flood plain zone approaches the flood problem preventively, seeking to minimize damages at the site and downstream by keeping

the flood plain relatively free of structures, fill and other detriments to

its sponge-like capacities to store flood waters.

Zoning utilizes the police power for its authority. As was noted earlier,

"The characteristic feature of zoning that distinguishes it from other policepower controls is that the regulations differ from district to district. For

this reason it can be used to set special standards for land uses in flood

hazard areas . . . The important aspect of zoning is that it can be used in

riverine or coastal areas to regulate what uses may be located in flood hazard

areas, where specific uses may be permitted, and how uses are to be constructed

or carried out." [24] The usefulness of the zoning power in planning hazard

amelioration is not, of course, restricted to floods. It could be equally

well applied to areas vulnerable to extreme winds, earthquake activity,

tsunamis, dam break, or any of the other natural hazards to be considered.

Case law resulting from environmental planning is still fairly spotted,

too few cases and too many conflicting decisions to yet provide a basis for

164
generalization. Flood plain zoning has, in contrast, a respectable body of case law to which reference can be made for parallel judgments on other

hazard zoning.

It is important to remember that all zoning stems from nuisance law and

must show that it has its police power to protect the health, safety, and

welfare of the public. The general area of conflict in zoning regulation,

as in all environmental management, lies between the individual property

owner's right to enjoy the benefits of his property as he wishes, and the

need for regulation of the uses of private property to safeguard the community

as a whole. The courts require that, in the exercise of the police power, the

regulations be reasonable. It must be shown that the restrictions upon the

individual are necessary and effective in serving a proper public purpose,

and that the benefits accruing to the community are great enough to warrant

the degree of restrictions on the individual.

Planning which seeks to regulate land use to mitigate damages to the

community from natural hazards enjoys a great legal advantage in that it

addresses directly the charge of its responsibility for public health, safety

and welfare. Even this advantage will be overcome, however, if the test of

reasonableness is not met. For example, flood plain zone regulations which

deprive a property owner of almost all remunerative uses of his land in

marginal floodway areas with low probabilities of flooding have been overturned by the courts. The lesson for all hazard-related planning, thus, is

to match severity of the regulation to the sever ity of the risk.

If it can be shown that regulation by police power is being used where

condemnation and compensation for land taken for a public purpose is more appropriate, the courts will uphold the property owner. In flood plain

zoning cases the suspicion that regulation was being used to protect land

which a municipality wanted for water storage, parks, or wild life protection

has led the court to rule against the flood plain zone provision. In formulating land use regulation to ameliorate natural hazards, therefore, the

purposes of the regulation must be clearly limited to protection of public

health and safety. Other public values such as recreation, open space or aesthetics must be considered apart from hazard zoning lest the authority of

the police power be jeopardized.

There has been some speculation that the sophistication and detail of

the data needed to regulate land use in hazard areas would make the cost of

regulation unfeasible. Data collection and analysis is always expensive to some degree, and requirements on local government for new ranks of data,

analysed and applied by new methodologies to new programs will surely give

any local finance director pause. For this reason there is a strong case

to be made for state and federal assistance to enable local government to

fulfill the obligations imposed by higher authority.

Nonetheless, in the history of flood plain litigation, the demands of

the courts for data supportive of regulatory decisions has been moderate and

would not, of itself, constitute an obstruction to flood plain or other

hazard zoning. The requirements have incorporated historical data to support

flood high water level probabilities for the five year flood, etc.; benchmarks

placed at reasonable intervals to establish elevations; a well defined map;

a clear statement of prohibited and permitted uses as well as performance

standards for the uses; and some flexibility of administration provided for

nonconforming use, exceptional circumstance and the like. Similar stipulations have been customary in most land use zoning.

Flood Plain Zoning and Environmental Planning: Overview

As noted earlier, environmental planning and planning for land use as applied to natural disaster mitigation share many of the same concepts and

165
concerns. It is understandable that planners seeking more comprehensive,

more rational, and more interrelated bases for their plans, should favor an amalgamation of the two areas of concern. Kusler and Lee are no exception

when they "have repeatedly urged that flood plain regulations be part of

broader planning and regulatory efforts. Often it is desirable to preserve

flood plain areas and wetlands to serve a range of goals . . . These lands

are often rich in wildlife and other environmental values." [25]

Some caution is advisable here. If protection of the public from effects

of natural disasters is the goal to be pursued, then practical, if short range,

considerations dictate that current enablement measures in which there is

legal precedence and established criteria should be sought. "The constitutionality of very stringent regulations which prevent all structural development to protect broad environmental values is still unsettled." [26]

Environmental planning and resulting legislation has been much criticized

for its single focus ecological determinism at the expense of social and

economic values. Richard L. Meier, in a much-cited article, states: "A more serious error in the positions taken by aggressive advocates of 'the

environment as it should be' is their i nsens i ti vi ty to social justice, a position that will do their cause great harm over the long run." [27] Hazard

amelioration planning, although in its infancy, has shown an excellent beginning in avoiding this pitfall.

Flood protection zoning, both coastal and riverine, has been built on a solid base of equity, reasonable probability and comprehendab 1 e risk factors

correlated with appropriate protective requirements. The excellence of the

grading codes achieved by the City of Los Angeles were only attained when the

hazards were evident and public pressure supported the necessary regulations.

And, a final example, the pioneering study for earthquake safety in the City

of Long Beach [28], has provided a technical framework to improve rational

decision-making in the public sector. It formulates "a code that is not

handed down by edict of an elite technical community" [29] but provides

instead "a means by which representatives of the lay citizenry can establish

or modify code limits recommended." [30]

Planning of any kind, to be effective, must achieve popular and

political understanding and support. Laws and ordinances which attempt to

force the populace to do what technical experts believe to be good for them

will be difficult to enforce and will not long endure [31]. Costs and benefits, those who pay and those who enjoy, must always be considered in the

conflict and tradeoff process of the political arena. A continuing sensitivity to these vital truisms will insure that future land use planning related

to natural hazards can be effectively implemented.

Examples of Current Regulations Which Consider Hazard

The following is a brief discussion of examples of current ordinances

or proposed ordinances which require consideration of a natural hazard. The

technology needed to effectively cope with the hazard problem is available

today and can be implemented as evidenced in current codes. Although the

ordinances presented in this discussion are for California governments, they

serve as examples of what can be done in the general area of land use planning

for disaster mitigation.

A . Seismic Safety Element - Section 65302.1 of the State of California

Government Code

The State of California has recently amended the Government Code,

relating to land use planning. This amendment requires that a seismic

safety element be included in the general plan of general law cities.

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Specifically, the law states:

"The general Plan shall consist of a statement of development

policies and shall include a diagram or diagrams and text

setting forth objectives, principles, standards and plan

proposals. The plan shall include ... A seismic safety

element consisting of an identification and appraisal of

seismic hazards such as susceptibility to surface ruptures

from faulting, to ground shaking, to ground failures, or to

effects of seismically induced waves such as tsunamis and

seiches." [32]

In support of the code, the California Council on Intergovernmental

Relations has prepared and submitted to all of the City and County Administrators interim guidelines. The guidelines state that the basic objective of

the revised State Code is

"to reduce loss of life, injuries, damage to property,

and economic and social dislocations resulting from

future earthquakes." [33]

B . Long Beach Existing Buildings Rehabilitation Ordinance No. C-4950

This ordinance has several unique features which are important to the

improvement of planning for hazards. They are presented as an example of one attempt to rationally interrelate zoning and building codes; and to show hew

the citizen and the elected representative, along with the career civil ser- vant, are interrelated and all considered part of the po 1 i cy- pi ann i ng- i mp 1 e

mentation system for hazard mitigation.

1) It couples land use planning with a building design ordinance

by requiring different designs in different zoned areas of the

City.

2) It required the City Council to make a decision on risk (in this case death risk) which makes any future council accountable for

its decision.

3) It prescribes performance standards for repairing or rehabilitating

structures

.

4) It provides for owner options of structure life and human exposure.

5) It provides a legal means for demolition of a structure at the end

of the selected structure life. This requirement is stated in the

title to the property.

6) Imperfections are recognized, specifically by requesting funds to

provide for soil dynamics investigation to upgrade the risk maps.

7) It provides for a uniform rating system by the Building Official

within a priority structure.

8) It allows for new construction materials or designs affecting the

damping properties, to be reviewed for purposes of adjusting code

values, thus lowering costs.

9) It provides a measure for dynamically analyzing structures and

sites. Thus, requiring building official personnel to be upgraded

in their profession.

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C . County of Los Angeles Interim Seismic Design Consideration for County

Buildings - Chapter 2-B-.07

This policy incorporates similar principles to those considered in the

Long Beach Ordinance, specifically importance of vital structures, cost risk,

life risk, and land use. In addition Geo! ogi c-Sei smol ogi c investigations are

to be obtained for all projects involving structures.

D. Senate Bill 479, (1971) to be amended by SB689 (1972), California

State Legislature

This bill would prohibit the construction of a school over the trace

of an active fault or below an area of slide, or in any other location where

the geological characteristics are such that the construction effort required

to make the site safe for occupancy is economically unfeasible. This is one of the first legislative attempts to prohibit construction near or on an "active fault."

E. Federal Legislation

Federal guidelines are currently being developed but in a very fragmented

manner. HUD, FHA, AEC, EPA, etc., are all developing guidelines but apparently without coordination with each other and/or local legislative jurisdictions

In many instances, the vocabularies differ and the meanings of key words such

as "active fault" have different connotations. These discrepancies should be

eliminated and the regulations or ordinances refined and integrated.

Summary

As explained in the foregoing discussion, there are land use planning

practices which consist of a series of surveys, analyses and policy decisions

starting at the general level and ending with specific regulations regarding

land use. As such, land use planning can and must encompass the full range

of pol i cy-pl anni ng- i mpl ementati on actions of any governmental unit engaged in

disaster mitigation.

As evidenced by environmental legislation and flood plain zoning

experience, as well as current hazard regulations, the "state of the art"

is sufficient to develop plans, programs and controls which effectively cope

with the problem of natural disasters. These plans, programs and controls

must, however, be prepared by engineers and planners in coordination with

professionally qualified governmental personnel; and presented in a manner

which both the political and citizen groups can understand and which can economically and effectively provide safety from natural hazards. Only then

will it be possible to provide the necessary enforcement required for disaster

mitigation.

VI. An Approach to Seismic Impact Assessment [34]

The following discussion presents a methodology for preparing a seismic

safety element for a General Plan similar to that utilized in the preparation

of environmental impact statements. This same methodology can be applied to

the other natural hazards, flood, wind, landslides, etc.

Preparation of A Seismic Impact Assessment

Seismic safety requires a consideration of the hazards associated with

168
earthquake in terms of the location of new structures and facilities, the

rationale and criteria utilized in design, an evaluation of existing structures and facilities, and the efficacy of existing codes and ordinances.

The following basic modes of failure that can occur during an earthquake in

both open and developed areas are to be considered.

a. Ground Shaking (addressed primarily by building code development

and enforcement)

• failure of structures due to shaking

• foundation failure beneath structures due to soil bearing

f a i 1 u re

t differential settlement of structures due to soil compaction

b. Permanent Ground Movement (addressed primarily by planning and

zoning code enforcement)

• landslide

• fault break through ground surface

• liquefaction of subsoil

• general land subsidence or lurching beneath structure

c. Water Forces and Flooding (addressed primarily by planning and

zoning code enforcement)

• failure due to tsunami or seiche

• flood due to dam break

To properly consider these failures and to balance the risks between the

failure modes, a specific sequence of acts must be accomplished which will

result in the development of a seismic element from which to make planning

pol i cy deci si ons .

A . Statement of Objectives

As a first step each jurisdiction must set forth its objectives regarding the planning element related to seismic hazard. The objective statements

should define the boundaries of the area to be evaluated and the degree of

sophistication to be incorporated into the evaluation. A plan of action

should be developed and the technologic possibilities of achieving the

objectives analyzed.

B . Preparation of an Emergency Plan

Since earthquakes occur without warning an emergency plan should be of

the highest priority in the preparation of a seismic element. The plan should

contain preparedness measures, as well as a response and recovery plan. If

prepared immediately, the plan will not have the benefit of the completed

hazard study, but it can become the basis of an improved emergency plan which

should be developed upon completion of the seismic element. Early preparation

of an emergency plan will serve to mitigate the effects of a disaster, even though formal programs have not been fully implemented.

169
C . Examination of Engineering Geologic and Soil Characteristics

Some State agencies, agencies of the Federal Government, and private

firms are actively developing seismic and geologic data necessary to prepare

a seismic element. This data (including surface and subsurface maps) must

be collected and examined in order to establish the geologic environment and

the related soils mechanics and dynamic properties of the subject area.

D . Development of Seismic Intensity Risk Maps

The results of hazards analyses are generally presented in the form of

risk maps, which portray the type and degree of hazard represented in a given

geographic location. Frequently, however, risk maps are prepared for large

regional areas which may not take into consideration local characteristics

and local hazards. Further, they are frequently prepared for undeveloped

areas on the tacit assumption that the developed areas are not exposed to the same risks as undeveloped areas. Micro-regional seismic risk mapping is

therefore required in order to develop a rational element for the general

plan of any local jurisdiction.

The risk maps are developed from the performance of a seismic risk

analysis. The results of this analysis, utilizing statistical techniques,

the geologic and soils characteristics, and the past earthquake history are shown on appropriately scaled maps divided by grids. Matrices accompanying

the risk maps show earthquake intensity probabilities for each map grid or cell. The decision matrices identify the probability of failure for each of

the nine failure modes in each cell. Since the earthquake hazard is directly

related to the numbers of people and structural types exposed, each decision

matrix must relate the risk of earthquake induced failure to structural life,

building importance or occupancy and economic factors.

E . Coordination of Risk Levels

At this point, the seismic impact analysis is performed based upon the

risk of failure in each mapped cell for the selected alternative modes of

failure considered important.

Failure of structures due to ground shaking and foundation failure due

to soil compaction or soil bearing failure are generally associated with

building codes. As such, a seismic impact analysis must include an assessment

of the potential hazard for both existing and new structures based upon an evaluation of the building code and the earthquake resistant capacity of

existing structures.

Structural failures due to fault rupture, landslide, liquefaction of

soil, land subsidence and lurching, tidal waves and dam break are generally

associated with zoning ordinances. In this area the impact analysis should

include an assessment of the current zoning regulations and planning policies

as related to the general physical land use elements; specifically, housing,

transportation and circulation networks and community/public facilities.

In its combined form the risk analysis can be graphically illustrated

as a three dimensional matrix (Figure 1). Each cell from the Community

Seismic Risk Matrix may contain several alternative probabilities of earthquake intensity and risk levels which must be correlated with all of the

various structural types located in the cell area. Based upon this impact analysis it is possible to assign risk levels for

loss of life and property in each cell area. Since the hazard in each area depends upon the various failure modes considered, as well as the selection

of risk levels, all are to be shown on hazard or risk maps. This will present a comparative hazard for each area.

170
FIGURE 1 COMMUNITY SEISMIC RISK MATRIX

171
The seismic impact analysis can be made with the aid of a seismic impact

matrix. The matrix would include on one axis the various modes of failure;

and alternative risk decisions based upon time and probable death risk or probable economic loss. The other axis of the matrix would identify the

various physical elements and structures which would be affected in the event

of a failure. Old, new and proposed structures are to be considered. This

system provides a format for a comprehensive review to remind the investigators of the variety of interactions that might be involded. It also aids

the planners in identifying alternatives which could lessen the impact of an earthquake. A typical impact matrix is presented in Figure 2.

F . Preparation of the Seismic Safety Element of the General Plan

The actual seismic safety element should include the risk maps which

establish limits on development and redevelopment based upon the probability

of seismically induced failures. In addition, the risk maps should identify

the risk exposure for those already developed areas within the Government

uni t

.

The test of that section of the general plan containing the seismic

safety element should include an assessment of the impact analysis and a discussion of the individual critical areas. The discussion should cover

the f ol 1 owi ng points:

1) A description of the proposed action including information and

technical data adequate to permit careful assessment of the

impact.

2) Alternatives to the proposed action.

3) A discussion of the social and economic impact of each proposed

action.

4) The relationship between short term and long term programs.

Based upon the risk and impact analysis, appropriate program alternatives

designed to mitigate the disaster effect of future earthquakes can be incorporated in local planning policy. A typical set of programs may include:

1) A program for improving the emergency plan based upon an understanding of the risks, gained from the results of the

completed seismic impact analysis.

2) Development of a means of equitably rehabilitating or eliminating

high risk hazards associated with existing developments and establishment of various capital improvement projects aimed at

eliminating high risk situations.

3) Development and implementation of inspection and evaluation

programs for evaluation of old structures.

4) Implementation of appropriate building code and zoning ordinance

revisions and the revision of other associated regulations.

5) Performance of more detailed seismic, soils, and geologic

investigations of problem areas where required in order to

gain confidence and minimize hazard.

6) Implementation of appropriate programs for protection of existing

critical facilities against the earthquake hazard.

7) The design of a seismic monitoring instrumentation program to

172
Cxdn.ine RISK DECISION MATRIX for each failure

norip and identify RISK FACTORS considered important .

2. For each of the important RISK FACTORS identify

the PHYSICAL ELEMENTS EXPOSED and place a slash through the appropriate box.

3. In the upper left hand corner of each box with a slash, place a number from 1 to 10 which indicates

the magnitude of the possible impact in terms of deaths; 10 represents the greatest magnitude and 1, the least. In the lower right hand corner of the box

place a number based on the same scale indicating

the possible ntagnitude of the impact ir terms of economic loss.

FIGURE 2 INFORMATION MATRIX FOR SEISMIC IMPACT ASSESSMENT

173
provide data for the continual upgrading of knowledge necessary

to more accurately assess risk.

8) Development of a program for updating the various parts of the

seismic element as more and new data become available.

Summary

The current state of the art will permit the preparation of a seismic

element utilizing the procedure outlined above. It is understood that our knowledge of seismic effects is neither complete nor perfect; and that

uncertainties do exist. However, objective analysis which incorporates all

available data and knowledge should provide a better and more rational

foundation for a decision; even though the decision may remain one of

muddling through.

VII. Conclusions and Recommendations

It may be concluded from the foregoing discussion that the process of

achieving an effective system of planning for disaster mitigation is one that

is frought with many difficulties. It is in this area that the general stateof-the-art needs to be upgraded. It is possible to improve our codes by

applying existing knowledge in a manner which all parties involved can com- prehend. To make no provisions in the codes, because we cannot accurately

predict a. disastrous event, is to make no^ decision and thus, reject the logic

of probabilities and the knowledge gained from science.

Regulations cannot be perfect and accurate for al

1

situations. Thus,

they must: (1) reflect the concept of risk and uncertainty; (2) be dynamic

in allowing for improvement through increased understanding; (3) be rationally

interrelated and tied to a plan which considers various disasters as key

elements; (4) be based on a logic which the legislator, citizen, and executive

(administrator) can fully comprehend, and thus allow for effective participation in the decision making process.

In order then, to make effective progress toward improving our condition,

it is necessary that new information and knowledge be utilized. The following

is a list of recommended actions for Federal, state and local levels of

government.

Federal Government

1. Develop criteria for establishing levels

guidelines of good standard practice for

achieve various levels of risk, and make

of risk, compile and maintain

engineers and planners to

building owners aware that

current practice involves risk to the owner,

Provide a clearinghouse for dissemination of engineering design criteria

of regulatory measures and administrative processes to assure implementation at the Federal and local levels.

Provide coordination of planning processes to assure reasonable consistency among states. Specifically, prepare a planner's "handbook" for

disaster mitigation which formulates elements within the comprehensive

plan to include:

(a) Guidelines and procedural steps for preparing disaster elements,

such as California's Seismic Safety Element.

174
(b) Regulatory instruments and procedures for implementation of

controls, such as "Earthquake Hazard Regulations for Rehabilitation

of Existing Structures Within the City," (Subdivision 80, Long

Beach Municipal Code).

4. Coordinate disaster mitigation research and disaster investigations.

5. Function as data bank and actively disseminate information to those

concerned. Specifically,

(a) The Federal Government should undertake a comprehensive program

of disaster risk mapping appropriate to the needs of local

planning and regulatory bodies, the design and building professions,

and industry practitioners.

(b) In so doing, it should make a feasibility study of how to undertake

an inventory of buildings that would provide data on the type of

occupancy, type of construction and other details of construction

needed to make viable potential risk evaluations.

(c) Concurrently, there should be a study to establish the damage

probability of the buildings for various kinds of hazard:

Engineering organizations with experience in earthquake resistant

design should be utilized for this study.

(d) Data concerning the probability of different levels of hazard occurrence must be compiled and made available through data

banks. The form and detail of the necessary data are described

in other recommendations appearing in this report. It is essential

that initial efforts using simple (and possibly crude) measures of

hazard be completed before more detailed compilations are undertaken .

(e) Data must be developed concerning the many less immediate and often

intangible costs of disasters: Loss of productivity, loss of tax

base, psychological impacts on individuals and community, etc.

Attempts to express all such costs in terms of dollars should be

avoided; for some studies, for example, the choice of the best

alternatives may be insensitive to any dollar value of human life.

6. Employ financial grant and loan insuring programs to encourage enactment

of effective land use planning and building codes and their strict

enforcement. Specifically, the Federal Deposit Insurance Corporation,

the Federal Home Loan Bank Board, The Federal Housing Administration,

and the Veterans Administration should request that the amount of money

loaned or guaranteed be insured against Natural Disasters. Furthermore,

natural disaster hazards posed by old and poorly-maintained buildings

should be one of the important factors considered in Federally financed

urban renewal programs.

7. Immediately commence with programs to correct deficiencies in buildings

occupied by the Federal Government in order to assure the survival of

vital government facilities in times of natural disaster.

8. Incorporate hazard mitigation considerations in Government building

and leasing operations comensurate with local risks, including certification of Federal construction projects on an individual basis.

9. The Federal Disaster Assistance Program should be reformulated so as to provide incentives for disaster mitigation efforts prior to a disaster and to remove incentives for ignoring the risks associated with

a structure and site. Specifically, the Federal Disaster Assistance

Program should recognize the need to consider post disaster rehabilitation and replacements to be designed and constructed with a view towards

175
mitigating future losses.

10. The Internal Revenue Code should be reviewed to identify features that

would provide equitable tax relief to individuals incurring losses in

natural disasters and taxing procedures for insurance companies writing

policies on infrequent disasters.

State and Local Government

1. Establish minimum rules, regulations and enforcements to provide for

the safety of the public and building occupants considering the hazards

ofnaturaldisasters.

2. The legislative body adopting a building code should create by statute

or ordinance a "Review (or Appeals) Board" to hear appeals from the

decisions of Building Officials and requests for variances. The legislation creating the board should define its functions and require that

most, if not all, members must be licensed professionals.

3. States should implement criteria for organization, staffing, procedures,

qualification, and certification of personnel for regulatory bodies.

4. States in cooperation with the appropriate federal agencies should set

minimum standards with respect to land use planning, building codes and

emergency planning to protect vital emergency services during and after

the occurrence of natural disasters.

5. States should require that local land use planning and public regulatory

measures be coordinated to assure that adequate disaster mitigation

provisions are incorporated into comprehensive plans. In this regard,

local land use plans should include a section or element dealing with

natural disasters and other hazards faced by local jurisdictions including definition of acceptable risk levels.

Also, states should provide for the establishment of a coordinating

mechanism to assure that the requirements of locally-conceived plans,

codes and ordinances are consistent with accepted area-wide standards

as related to natural disasters.

6. Regulations for land use and building construction should include

provisions for evaluation of the safety of existing buildings against

all natu ral hazards

.

7. Locally developed land use plans should be coordinated with area-wide

and state land use planning, to assure that maximum effective use is

made of limited resources.

VIII. References

1. California State Office of Planning, California State Development Plan

Program , 1968.

2. Guttenberg, Albert Z., ASPO preface in HEM Directions in Land Use

Classification , ASPO Publication, Chicago, 1965.

3. Kent, T. J., Jr., The Urban General Plan , Chandler Publishing Company,

San Francisco, 1964, p. 18.

4. Chapin, F. Stuart, Jr., Urban Land Use Planning , University of Illinois

Press, Urbana, 1965, p. 349.

176
5. Banovetz, James M. (Ed.), Managing the Modern City , International City

Managers Association, Washington, D. C, 1971.

6. Murphy, F. C, Regulating Flood Plain Development , 1958, and Wertherrner,

Flood Plain Zoning , California State Planning Board, 1 942 , are but tv/o

early examples.

7. Goodman, William J., and Eric C. Friund, Principles and Practice of

Urban Planning , ICMA, Washington, D.C., 1968, p. 403.

8. , "Standard City Planning Enabling Act, (Model Legislation),"

published by U.S. Department of Commerce, 1928.

9. , "Building Housing Zoning Codes and Their Enforcement,"

University of California, Irvine/Orange County Chapter, American Society

of Public Administration, January 1971.

10. Friedmann, John, "Notes on Societal Action," Journal of the American

Institute of Planners , September 1969, p. 311.

11. Walker, Robert A., The Planning Function in Urban Government , University

of Chicago Press, Chicago, 1941, Second Edition, 1950.

12. Kent, T. J., Jr., The Urban General Plan , Chandler Publishing Company,

San Francisco, 1964, p. 18.

13. Chapin, F. Stuart, Jr., Urban Land Use Planning , University of Illinois

Press, Urbana, 1965, p. vi.

14. Sedway-Cooke , 400 Pacific Avenue, San Francisco, California 941 33 , Unpublished Memorandum to the County of Santa Cruz, California,

March 22, 1972.

15. Message to Congress, August 1970, Found in "Environmental Quality, The

First Annual Report of the Council on Environmental Quality," p. v.

16. Haskel , Elizabeth, "New Directions in State Environment Planning,"

Journal of the American Institute of Planners , July 1971.

17. Bosselman, Fred and David Callies, The Quiet Revolution in Land Use

Control , Publication of the Council on Environmental Quality,

Washi ngton , D.C. , 1971 .

18. Wise, Harold F., "More Than Planning," National Civic Review , May 1966,

Vol . LV , No. 5 , pp . 242-3.

19. , "The Role of Engineering Geology in Urban Planning," the

Colorado Governor's Conference on Environmental Geology, 1969.

20. , "The San Fernando, California Earthquake of February 9,

1971," a Preliminary Report Published Jointly by the U.S. Geological

Survey and the National Oceanic and Atmospheric Administration (NOAA)

Geological Professional Paper 733, and Comprehensive Final Report (now

in printing) by NOAA which was prepared by a Special Committee of the

Earthquake Engineering Research Institute - D. F. Moran, Chairman.

21. , "Wind Forces on Structures," ASCE Transactions , Volume

126, 1961, Part II, Paper No. 3269.

22. Wallace, McHarg, Roberts and Todd, "Regional Transportation Plan for

Colorado: Phase I - A Concept," January 1972.

177
23. Leopold, L. B., F. E. Clarke, B. B. Hanshaw, and J. R. Balsley, A Procedure for Evaluating Environmental Impact , Geological Circular

645 , U.S. Geological Survey, Washington, D.C, 1971.

24. Kusler, John A. and Thomas M. Lee, "Regulations for Flood Plains,"

ASPO, Planning Advisory Service, Report 277, February 1972.

25. Ibid, p. 65.

26. Ibid.

27. Meier, Richard L., "Insights Into Pollution," Journal of the American

Institute of Planners , July 1971.

28. Wiggins, John H., Jr., and Donald F. Moran, "Earthquake Safety in the

City of Long Beach Based on the Concept of Balanced Risk," September

1971 .

29 . Ibi d , p . xi .

30. Ibid, p. 1 .

31. One address to the problem of public indifference to safety regulations

to ameliorate catastrophe is seen in the "San Jose, California, General

Goals Statement" of April 1968. This statement, developed by citizen

groups, noted that necessity for a civil defense and disaster program

but added "Because there is ample evidence to show that there is a lack of community interest in the need for and the purpose of a civil

defense and disaster program . . . there is a need to include public,

private and volunteer groups in the interpretation of these programs."

The political effectiveness of citizen participation in decisions

relating to public safety should never be overlooked.

32. State of California, Government Code, Section 65302, enacted in 1971.

The statute went into effect on March 5, 1972.

33. California Council on Intergovernmental Relations and the Governors

Earthquake Council, "Suggested Interim Guidelines for the Seismic

Safety Element in General Plans," July 14, 1972.

34. This Section is a revised version of a PAS Memo entitled, "Procedure

for Developing a Seismic Safety Element for the General Plan," by

William J. Petak , soon to be published by the American Society of

Planning Officials.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards , August 2 8-September 1,

1972 , Boulder , Colorado (Issued February 1973).

178
ARCHITECTURAL APPROACHES TO HAZARD MITIGATION

by

Ernest C. Hi 11 man JrJ

and

Arthur E. Mann, AIA^

I. Intro duct ion

This review paper presents an evaluation of current practice as it

relates to Architectural Approaches to Hazard Mitigation. The subject matter

endeavors to treat the topic as stated in the program announcement for the

forthcoming Workshop sponsored by the National Bureau of Standards. The

scope of this paper is:

"Building forms, spatial distributions, and interior designs

can be adapted to natural disaster hazards as part of the design to

provide a functional environment in harmony with the natural environment. The review should evaluate paradigms, procedures, and criteria

for synthesis of external and internal forms which provide effective

function and benign interactions with earthquakes, extreme winds,

explosives and similar causes of disasters. Methods should be

reviewed for encouraging architectural approaches to disaster mitigation at the programming and schematic design stages."

Although the program is intended to be of national scope, the principal

subject presented relates to building hazards caused basically by earthquakes

and, for this reason, most of the information revolves about the current

situation in the State of California where the occurrence of earthquakes and

earthquake-resistant design are more prevalent.

The design of structures for wind forces can be quite complicated and

current codes list wind forces which usually vary with the height of the

building. Designing a structure for the largest code force is the simplest

method; but, when designing larger structures, the determination of wind

forces by modeling or by a mi cro-cl i matol ogi st may result in considerable

construction economy. The shapes of the building could be so arranged that

the least surface area would be normal to the "prevailing wind" direction.

Sheltered conditions may be a factor. (Paper 4 will review "Procedures and

Criteria for Wind Resistant Design.")

Explosion hazards are rather unusual situations and are somewhat undefinable but, in general, good earthquake resistant design and attention to

accepted fire and panic practices would provide a substantial deterrent to

such damage. An approach to hazard mitigation in blast design would be to

provide a structure that would not fail progressively due to a local failure.

Because of the regional aspect of earthquakes, most of the references

to current practice now appearing in the Building Codes and research documents

are of California origin.

President, Hillman, Biddison & Loevenguth, Structural Engineers, Los Angeles

Sr. V. President, Daniel, Mann, Johnson & Mendenhall, Architects, Los Angeles

179
This paper will be presented in three main parts under the following

headi ngs :

• Current Architectural Practices and Policies with Respect

to the Design of the Structural Portions of a Building.

• Current Architectural Practice as it relates to Design

of the Non-Structural Interior Elements of a Building.

• Summary of Recommendations to Further Implement Building

Hazard Mitigation.

In the Appendix of this paper, references to various Codes, Rulings, and

research data by California engineering societies and manufacturers' manuals

are listed.

II. Current Architectural Practices and Policies with Respect

To the Structural Design of a Building

Architect-Engineer Consultant Relationship

On large, major structures it has been the current general policy of

Architects to engage a Consulting Structural Engineer to design and prepare

the structural plans for the project. In some areas, if warranted by a steady volume of commissions, large architectural firms have established inhouse engineering capability to perform this service.

A majority of Architects engage structural services on the more modest

projects and even on small structures. Many projects, however, are developed

with a minimum of structural assistance, perhaps only partial service. This

is most unfortunate because the Archi tect- Engi nee r relationship is possibly

the most important parameter in achieving good structural design as a means

of promulgating hazard mitigation.

Virtually every state in the nation, through registration laws, requires

structures to be designed by Architects or Civil Engineers and the State of

California further restricts design to Architects or Structural Engineers

(Civil Engineer plus Structural Certificate) for all public school buildings.

By custom, and justifiably, the Architect is generally engaged as the

prime professional, as he is basically trained and qualified in planning and

esthetics and is legally permitted to design all facets of a building project.

In seismically active regions, the role of the Architect becomes more complex.

Inflation and the resultant demand for economy in construction compounds his

design problem. Very often his client may not accept the esthetics of an "earthquake-safe" design, nor permit spatial interruption that might be re- quired to provide a high level of lateral load resistance.

Except for unusual structures, static vertical load design is not mysterious or complex, but can become increasingly complex when lateral force

design must be recognized, and more particularly so when the project is located

in known or suspected seismic regions. The need for a close relationship

between the Architect and Structural Engineer is generally well recognized by

these professionals in California.

A vivid historical picture of earthquake damage and loss of life dates

back to the San Francisco earthquake and fire of 1906.

Of necessity, accelerated interest in seismic design followed the Long

Beach earthquake in 1933, and for the first time, legislation was enacted to

require recognition of this serious building hazard. Apparently the lesson

180
of proper structural design was not learned until 1933. Countless hours of

time and research by engineering society committees and academic circles has

brought us to the current "State of the Art."

Much of this "art" and current code provisions were generated by the

Structural Engineers Association of California which, in 1959, prepared the

document "Recommended Lateral Force Requirements and Commentary." Many

revisions have been made, the last being Appendix F, which includes 1969,

1970 and 1971 revisions. This "blue book" has formed the basis for seismic

design since 1960 and has strongly influenced seismic design throughout the

worl d

.

In 1971 a new document, "Report of the Ad Hoc Committee on Direction

Study - Seismic" was published in the Annual Proceedings of the Structural

Engineers Association of California. Included in this document is the

subject "Risks Not Considered by Code," which points up two major areas of

design process, namely, siting of the project and basic architectural con- cepts [7].

The complexity of current seismic design virtually demands that the

Architect and the Consulting Engineer have an extremely close relation,

particularly in the initial design or schematic phase of the project.

Currently this relation is not always as close as it might be, nor does it occur as early in the project planning as it should. All too often the

architectural design concept is crystallized before all structural problems

are known and consequently the structural design becomes a compromise involving esthetics, economy and good seismic-resistant design. This situation

seems more prevalent in the small to medium sized projects where, for some reason, the importance of the Architect-Engineer relation is discounted. The same consulting fee can generally cover the complete service when the Engineer

becomes involved at the outset of the project.

Geology and Soil Stability Consideration in Site Selection

The selection of a building site by an Owner can have a great impact on the design and ultimate cost of a project. Unfortunately, the selection of

urban area sites and, to a substantial degree, sites in outlying districts

of metropolitan population centers is not very flexible. Where a choice is

available, preliminary geologic studies are occasionally made in addition to

the customary soil stability and capacity investigation in order to select

the most favorable site. It is essential therefore that the Architect and

Engineer be involved at the outset of the project to aid in site selection,

but currently this early relation with the Owner seldom occurs.

Where site selection is inflexible, current practice is to adjust the

design within economically feasible limits to provide reasonable safety

against geologic hazards.

Soils geology is a relatively new profession, at least in regard to its

application to building design. It will take some time for data banks to

accumulate enough information for general use.

In general, building regulatory agencies have no provisions requiring

geologic studies of a building site. The State of California Education Code

does require that geologic investigation be made prior to the acquisition of

a school site and for additions to an existing school facility where no investigation has been previously made [1].

An ordinance adopted by Los Angeles County in 1970 prohibits the con- struction of a building directly over a known fault trace, or within a certain distance. There are evidently many fault traces not located and

require trenching to uncover, if possible [2]. This is a step in the right

181
direction but does not solve all the problems that should be considered in

earthquake-resistant design. The County Code also includes a broad provision

relating to flood hazard and earth fills along with reference to geologic

hazards, but this is aimed primarily at subdivision developments. Alameda

County (San Francisco area) has a similar ordinance. Article 5 of the

California State Planning Law has similar provisions for the same purpose.

On July 14, 1972, the Governor's Earthquake Council elaborated on this law by

preparing a document entitled "Suggested Interior Guidelines for the Seismic

Safety Element in General Plans" [2].

In March 1972, Los Angeles County commissioned Woodward-McNeill & Associates to prepare a comprehensive study on "Geologic and Seismic Consideration for Land-Use Planning." This study will produce information on two

essential items, namely: (a) Land-Use Maps, which will delineate areas where

various earthquake-resistant features should be provided for building; and

(b) Safety Guidelines (Code Provisions) for the location, design and construction of structures in the County [3].

Currently, subsurface soils investigations which are made on a large

percentage of projects include some geologic data, but in general the "State

of Geologic Art" is such that many potential geologic problems cannot be

accurately predicted. The San Fernando earthquake of 1971 in Southern

California where considerable damage caused by earth faulting and surface

rupture occurred, demonstrated that much of this ground failure would have

been difficult to predict.

Structural Framing Systems v.s. Architectural Design

Subject matter presented relates primarily to structural system design

for lateral forces. Structures should be designed for vertical loads with

proper consideration given to the effect of lateral loads. The structure

should then be analyzed for lateral loads (wind or seismic) and connections

and members modified to accommodate the combined loading. Consideration must

be given to the drift of the building to minimize non-structural damage.

There are two basic design systems to provide lateral force resistance

in a structure: (1) shear wall, or braced frame systems, and (2) moment

frame systems. These systems may be used in combination if proper consideration is given to their interaction. Codes recognize the differences in these

systems and assign lower seismic loading to the moment-resisting frame.

The architectural planning problem is generated in endeavoring to provide

open floor areas uninterrupted by permanent structural walls or extremely

large columns. Also, the esthetic design of the exterior facade may demand

a high degree of fenestration. These two criteria generally set the pattern

for the structural system. The Architect and Engineer should work to develop

a symmetrical framing system that will meet architectural requirements.

Current design practice generally utilizes moment frames exclusively in

structures more than 15 stories high. These can be of structural steel or ductile concrete frame design. Frame structures obviously provide a maximum

latitude in architectural design.

Structures under 15 stories high and particularly those in the 2 to 10

story range create the broadest spectrum of shear wall v.s. frame design as shear wall structures in lower rise buildings are general.ly more economical

provided the architectural planning can accommodate them. It is in this

range that current knowledge and code provisions are mu 1 ti -faceted . It is

also in this area where the ingenuity of the Architect is taxed to the utmost

in developing effective tenant space and producing an esthetically acceptable

building. If minimal interior and/or exterior wall elements only are available, current design criteria provide variable lateral load factors when

182
frame action is also included. The architectural design then must be balanced

with economy and good seismic design which translates into employing the

maximum amount of wall consistent with function and appearance.

Code lateral force factors now can vary as much as 50 per cent, when

building height and amount of walls vary. Unfortunately these theoretical

reductions do reduce the reserve strength in a structure and current Code

factors do not take into account irregular floor plans and the mixture of

different building materials. An example would be an "L" shaped building

of variable height, using steel, masonry, concrete and wood throughout the

structure. Here, the absence of horizontal symmetry, vertical regularity

coupled with the marriage of different materials creates serious architectural

design problems with respect to inherent safety. Reserve strength available

in older structures has been constantly reduced by development of lightweight

building materials, both structural and non-structural and the use of lightweight and relatively flexible exterior architectural facades. Hazard mitigation is inherently improved when architectural planning accepts the

assistance of good engineering judgment used within the framework of existing

Building Code provisions.

Due to the current gap between scientific data and its practical

application to building design, reliable knowledge of actual lateral loading

and resultant building response is still limited. Therefore until more capability is generated, the Architect of necessity must give serious con- sideration in spatial and esthetic planning to simplify lateral force problems.

This is an enormous order, considering the fact that he is in competition for

commissions awarded many times on the basis of exotic designs and severe economy demanded by clients who do not completely comprehend (except for the

first few days following a disaster) the serious need for recognition of hazard

mitigation.

Selection of Exterior Facades

Building perimeter enclosure walls can be of the following types:

• Concrete walls, piers and spandrels poured mono 1 i th i ca 1 1

y

with the frame.

• Pre-cast concrete panels of architectural concrete with or without bonded stone veneer.

• Masonry filler type walls.

t Stone or ceramic veneer applied on concrete or masonry walls,

t Metal and glass curtain wall panels.

• Wood or metal studs and plaster.

The type of exterior facade selected is based on the architectural design and

structural requirements, with economy being an important factor. The integration of the selected facade with the total structure should provide for an infrequent, but full, fury of nature. Monolothic concrete enclosure elements

generally act also as lateral force resisting elements. Precast attached

units may also be used to resist lateral forces, but generally in low rise

structures. In these cases, the structural design requirements may have a substantial impact upon the architectural design. More architectural freedom

is permitted when the other non-lateral force resisting types are selected.

Current practice and code provisions establish criteria for lateral

force design and connections. These current methods to date have performed

rather satisfactorily with virtually little history of failure in recent

earthquakes. In addition to design for wind (15 to 30 psf) or seismic

183
loading (.2W) normal to the wall, specific requirements are established in

codes for connections. Connections to the frame shall resist twice the

weight of the panel. Connections and joints shall allow a relative movement

between stories of not less than two times the story drift or 1/4 inch, whichever is greater. Connections must have sufficient ductility and rotational

capacity to preclude fracture of concrete or brittle failure at welds. Design

shall also allow for story drift in the plane of the panels. Obviously, these

design provisions place an extra burden upon the Architect in that his design

must accommodate structural features.

Equivalent design requirements are used for "curtain wall" panels and

general practice follows standards set forth in good detail in the "Metal

Curtain Wall Manual" published by the National Association of Architectural

Metal Manufacturers.

Attachment of concrete, masonry and ceramic veneers is specified in

detail in Chapter 30 of the Uniform Building Code, and also in corresponding

chapters in the major California city codes as well as the California State

Code (Title 21) which is somewhat more stringent. The systems currently in use have performed satisfactorily. Adhesive methods have experienced some

mi nor prob 1 ems

.

Current glass and glazing criteria set forth in Chapter 54 of the Uniform

Building Code and corresponding chapters in local codes appear to provide

adequate safety against lateral force loading, although some breakage has

been experienced in stronger earthquakes. Investigation of these failures

usually reveals improper installation. However, some control over glass pane

clearances for distortion in the plan of the window wall should be established.

In regions of high wind velocities, more attention must be given to strength

and particularly deflection characteristics of glass panels normal to their

surface. The elimination of potential glass breakage is probably the most

important item in architectural hazard mitigation.

Current Research by Various Agencies

With respect to earthquake hazard mitigation, the Seismology Committee

of the Structural Engineers Association of California, functioning in close

relation with the Seismology Committees of the four local area Structural

Associations, has exerted the greatest influence on earthquake-resistant

design. Engineering Departments of the major colleges in California provide

substantial input as many faculty members serve on these committees. Similarly, staff members of research departments of the various building code enforcement agencies perform a similar function. Record of accomplishment has been

very good, despite the fact that practically all service is provided without

remune rati on

.

Occasionally some industry research is generated but usually occurs when

a proprietary item of a manufacturer needs acceptance by a building code

agency. This research is generally performed by a commissioned consulting

engineering firm.

Tlie Applied Technology Council, a non-profit corporation sponsored by

the Structural Engineers Association of California has been established to

provide the primary function of implementing current technological research

development into active building design practice. The Council will operate

with a limited paid staff who will define needed projects and award commissions

on a priority basis to qualified agencies. Other professional design societies

will probably be contacted and invited to join or provide input.

184
III. Current Architectural Design Practices and Policies

With Respect to Interior Non-Structural Elements in a Building

Archi tect- Engi nee r Consultant Relationship

Interior elements of a building such as ceilings, light fixtures and

partitions installed by historically conventional methods offer no special

hazard except in regions subjected to earthquakes. In California, relatively

little engineering attention has been given to this segment of construction

until recent years. It has become necessary that lateral force design attention be given to these elements in order to eliminate hazard to occupants and

to minimize property damage. As part of their engineering service, many

consultants assist architectural clients by reviewing details of these

"non-structural" items.

Current Design Criteria for Ceilings, Fixtures and Partitions

Except for free floating "cloud" type elements, suspended metal lath and

plaster ceilings on a steel channel system create a minimum hazard because of

their inherent stiffness and ability to transfer load to perimeter structural

elements, provided the ceiling system is properly wire tied to the structure.

Integrated ceilings consisting of light metal T-bars , drop-in acoustic panels

and light fixtures require greater structural attention. California State

and some local Building Codes have established minimum lateral bracing criteria for ceilings of this type. In general, these Codes require that the

ceiling construction be designed to resist or successfully transfer a load 1 pound per square foot minimum, or 20% of the total ceiling weight to structural elements plus lateral loads from partitions that are not independently

braced. Additional independent lateral and vertical support is required for

light fixtures equal to 100% of the fixture weight. The Ceilings and Interior

Systems Contractors Association has prepared recommended standards which present in more detail the requirements of current building codes. This standard

pattern will probably be adopted [4, 5, 6].

Light fixtures other than those set in integrated ceilings, such as pendant type are required to have ball joints permitting large arc free swing

or have tube hangers of sufficient strength to safely resist bending stresses.

However, free swinging fixtures require special attention, so as not to allow

the fixture nor its stem to hit adjacent portions of the building during an earthquake thereby resulting in destruction of the fixture and hazardous

fal 1 i ng debri s .

No specific design provision appear in building codes for lateral support

of interior partitions except that, in general, partitions should be braced

at the top to resist a lateral load of 20% of the tributary weight of the

partition. This load can be transferred into the ceiling if designed to

receive partition loads or braced to the structural framing above.

In the 1971 San Fernando earthquake, plaster ceilings and walls in

primary exit stair towers were damaged due to drift in several high-rise

structures. The falling of small portions of plaster is somewhat hazardous

to occupants during evacuation, but the more serious consequence can be

panic in the minds of the layman, who would assume structural failure was occurring. Stair towers are often the stiffest elements in a building and

yet on many occasions experience great damage. Stair towers are absolutely

vital for evacuation and for access of rescue teams. The towers must be

designed to remain intact.

185

493-610 O - 73 - 13
Mechanical and Electrical Equipment and Elevators

This subject is mentioned here only to point out that it is a very major

item of "non-structural" hazard in a building. This subject is presented in

a separate Paper which, no doubt, will include the importance of piping, air

conditioning equipment and ductwork support and anchorage. Elevators received

considerable damage in the San Fernando earthquake and considerable attention

is needed to mitigate this hazard. ^.

Studies are now underway by manufacturers and Code agencies to correct

deficiencies which were experienced in the earthquake. Counterweight security

' and guide rail alignment are particularly important factors.

IV. Summary of Recommendations to Further Implement

Building Hazard Mitigation /

Successful design of a project, specifically with respect to hazard

mitigation virtually demands a close working relation between the Architect

and the Consultant. Architect and Engineer associations through joint action

should encourage closer working relations particularly in the early stages of

project planning.

Starting with the architectural student, schools of architecture should

be encouraged to develop courses in all types of major and even minor hazard

mitigation, even though design and planning capability is very important.

By expanded public relations the Architect and Engineer should inform

the public of inherent hazards to buildings in seismically active regions and

of the importance of accepting less spectacular design concepts for projects

in these localities. In addition tO safe major building design, professionals

should develop a program of general education of the public on minor but

disturbing earthquake hazards in the home or office. Anchor tall objects to

the wall and improve closet and cupboard door hardware to prevent the falling

out onto the floor of glassware and medical supplies that very often result

in foot hazards or injury.

As more scientific research is successfully translated into practical

application, the Engineer-Consultant should produce techniques that will

develop better and perhaps more economical designs for exterior facades of

buildings. Development of special materials and proprietary items of manu- facturers should be encouraged by the design professionals. More intensive

programs of fenestration design for earthquake and, more importantly, wind

resistance is needed.

Greater emphasis must be placed on the importance of geologic study of

building sites as it relates to ground motion to supplement the more conventional soils investigation which is now aimed primarily at foundation design.

The Architect and Engineer must develop a closer relation with the Geologist

so that they will better understand the needs of the design professional.

Architect and Engineer code committee activities at present are mainly

confined to reviewing proposed building ordinances and are usually defensive

in nature. Instead, Architect and Engineer associations should work diligently

to find methods of financing, including public funding, for applied research

and technology related to all phases of building hazard mitigation. These,

and continuing studies should be carried out as expeditiously as possible so that building regulatory agencies, who are responsible for building code

changes, can be assisted and influenced by the design profession. This important activity is necessary to minimize the "panic factor" that results from

186
public and political pressure on building departments to take drastic action

immediately following a natural disaster. These quick, violent moves tend

to produce ultra-conservative requirements that have a great impact on building construction cost.

Although residential buildings receive little professional structural

design attention, they represent a large segment of construction dollar

volume and house a large segment of the population. The San Fernando earthquake created extensive damage to residences. Better arbitrary standards of

design in building codes should be provided, paying particular attention to

split-level homes, need for shear elements in the main building and garages,

and design and installation of large glazed wall openings.

In conclusion, the Architect as the prime design professional, along

with his Consulting Engineers, must face building design and hazard mitigation

in perhaps a two-fold attitude. First, buildings must be designed to protect

all occupants from injury or loss of life. This philosophy is obviously

mandatory and fundamental. Second, design approaches must include a favorable

balance between property or physical damage to a structure with respect to

initial cost. Even remote structural damage may be tolerated in extreme disasters if occupant hazard is nil. The profession should promote the increase

in awareness by the public investor or owner of this situation.

V. References

1. Section 15002 of California State Education Code. Revised July 11, 1972

by Senate Bill 689.

2. Los Angeles County Ordinance 10037. Los Angeles County Code Sections

308 & 309. City of Hayward, California, Code 306. California State

Code , Arti cl e 5 (65300) .

3. Proposal of Woodward-McNeill to Los Angeles County for Geologic and

Seismic Considerations for Land-Use Planning.

4. Design and Construction of Ceiling Suspension Systems. Office of

Architecture & Construction, State of California.

5. Rule of General Application RGA 3-67 Department of Building and Safety,

City of Los Angeles.

6. Recommended Standards for Seismic Restraint of Direct Hung Suspended

Ceiling Assemblies, Prepared by Ceilings & Interior Systems Contractors

Associ ati on

.

7. Ad Hoc Committee on Direction Study--Sei smi c , Structural Engineers

Association of California.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop S ponsored by the National Science Foundation

and the National Bureau of Standards, August 2

8 -S eptember 1,

1972, Boulder, Colorado (Issued February 1973).

187
PROCEDURES AND CRITERIA FOR EARTHQUAKE RESISTANT DESIGN

by

C. W. Pinkham*

1. Introduction

Purpose of Paper

This paper intends to (a) assemble the current status of procedures and

criteria for earthquake-resistant design of buildings, (b) indicate the

direction the procedures and criteria should take to improve the reduction

of earthquake hazards to buildings, and (c) establish priorities for future

research

.

History of Structural Design Criteria

The development of codes governing the design of earthquake resistant

buildings in the United States is synonymous with the development of such

codes in California, even though experience of severely damaging earthquakes

extends over many areas of the Nation. For example, the three earthquakes

centered near New Madrid, Missouri, in 1811 and 1812; the earthquake of 1886

centered near Charleston, South Carolina; and, the earthquake centered in

Prince William Sound, Alaska in 1964.

In the Commentary of their publication "Recommended Lateral Force

Requirements and Commentary," the Structural Engineers Association of

California (SEAOC) [1] provided a short history of the development of earthquake codes in California. It is reprinted here so that current provisions

may be viewed in their historical context:

"San Francisco was rebuilt after the earthquake and fire of

1906 under a code which provided 30 psf wind force to effect both

wind and earthquake resistance. In the years that followed, leading

structural engineers employed the concept of lateral earthquake

forces proportional to masses, but this simple Newtonian concept

did not find its way into codes until 1927 in the Uniform Building

Code and more extensively in 1933, following the Long Beach Earthquake.

The real impetus for extensive earthquake studies and research

resulted from the damaging effects of the 1925 Santa Barbara Earthquake. At that time, the U.S. Coast and Geodetic Survey was directed to make studies in the field of seismology. The work of

U.S.C. & G.S. in this field is continuing and is recognized worldwide. Shortly before the Long Beach Earthquake of 1933, the first

strong-motion seismograph was developed. Although the range of the

instrument was exceeded, the essential acceleration record of that

earthquake was obtained.

In 1927, the Uniform Building Code contained a section on earthquake provisions in the appendix with the following preamble:

"The following provisions are suggested for inclusion

in the Code of cities located within an area subject to

*President, S. W. Barnes and Associates, Los Angeles.

188
earthquake shocks. The design of buildings for earthquake shocks is a moot question but the following

provisions will provide adequate additional strength

when applied in the design of buildings or structures."

In 1928, the California State Chamber of Commerce recognized

the need for a building code which was "dedicated to the safeguarding of buildings against earthquake disaster." Studies under this

sponsorship included work by many of the leading structural engineers

of the State and resulted in a report which, while not adopted,

formed the foundation of codes that followed.

In March, 1933, the Long Beach earthquake destroyed many buildings in that area of the State including many public school buildings.

If the shocks had occurred during school hours, the loss of life

among the school children would have been appalling. Realizing that

much of the loss and damage could be avoided if the buildings had

been properly constructed, the State Legislature adopted the Field

Act that assigned to the Division of Architecture of the State

Department of Public Works, the authority and responsibility, under

the police power of the State, to pass upon and approve or reject

plans and specifications and to supervise the construction of all

public school buildings. Appendix A of the Rules and Regulations

adopted by the Division of Architecture required masonry buildings

without frames to be designed for a lateral force of 10% of dead

load plus a portion of live load. Other buildings were to be

designed for 2% to 5%. The lower coefficients were related to

higher allowable foundation loads. In 1937 the requirements were revised to make the coefficients 6% to 10% for buildings 3 stories

or less in height, or buildings without a moment-resistant frame.

Buildings more than 3 stories in height with a complete momentresisting frame had coefficients of 2% to 6% provided the frame

could resist 2% of the load. Again, the range of values were related to the allowable soil pressures. In 1941, the coefficients

used were 6% to 10% depending only upon the type of foundation

materials. Since 1953, the coefficient used is based upon the

equation (60 over N+4.5) where N is the number of stories above

the story under consideration. The present requirements are given

in Title 24 California Administrative Code,

In 1933, the Riley Act adopted by the California State

Legislature required all buildings except certain type dwellings

and farm buildings, to be designed for a lateral force 2% of the

vertical design load. In 1953, this requirement was revised to

require 3% for buildings less than 40 feet in height and 2% for

those over 40 feet in height.

In 1933, the Los Angeles Building Ordinance required a coefficient of 8% of dead load plus half live load. This was also required in the Uniform Building Code of 1935 on soils good

for 2000 psf or more in areas of highest seismicity (Zone 3),

with double this value for weaker soils.

In 1943, the City of Los Angeles recognized indirectly the

influence of flexibility on the earthquake design coefficients,

making the coefficient a function of the number of stories in the

building by the formula

ft n

^ + 4 5 where C = coefficient % of dead load

N = number of stories above the

story under consideration.

The maximum number of stories

was thirteen.

189
In 1959, when the height limit was removed

Angeles Code, this formula was modified to read

from the Los

N+0

4.6$

79TT 87 where S total number

the building

buildings of

less

.

of stories in

except S = 13

13 stories and

It wasn't until 1947 that San Francisco had anything more stringent than the Riley Act in its Code. A table of variable

coefficients was adopted, with maximum value for one-story buildings of 8% and minimum value for 30 stories of 3.7%, with variations

for soil conditions. These were applied to design vertical loads.

This prompted the formation in 1948 of a Joint Committee on Lateral Forces of the San Francisco Section, ASCE, and the

Structural Engineers Association of Northern California. This

committee, after several years' study recommended a code in which

the coefficients were related to the estimated or calculated

fundamental period of the structure.

C

= where K = 0.015 for buildings

= 0.025 for other structures

T = period in seconds

For

These coefficients

cent live load, with 50 areas .

For buildings:

othe r structures:

Cmax = 0.06 Cmin = 0.02

Cmax = 0.10 Cmin = 0.03

usually were applied to dead load plus 25 per

per cent live load for storage or warehouse

San Francisco, in 1956, adopted a variation of the recommendations of the Joint Committee:

where K = 0.02 for buildings

= 0.035 for other structures

T = period in seconds

For buildings: Cmax = 0.075 Cmin = 0.035

For other structures: Cmax = 0.10 Cmin = 0.04 "

In 1957, SEAOC formed a Statewide committee to develop a seismic code

that would be acceptable to structural engineers throughout the State. The

committee's final report in 1959 has become the basis for current codes most

frequently used by governing agencies. The base shear of the structure was determined by the formula V = KCW . The term Z was added by the International

Conference of Building Officials (ICBO) to the Uniform Building Code (UBC) to

account for seismic zoning. The factor C was specified as equal to 0.05 with

a maximum value of 0.10. K was introduced to account for the fact

historically, certain types of building systems had shown better performance

than others. For buildings, the value of K varied from 0.67 to 1.33. Specific requirements and limitations were also specified for individual portions

of buildings and for use of different construction materials.

From 1960 to the present, most of the changes to the SEAOC Code have

been directed toward specific requirements necessary to accomplish the degree

of ductility deemed appropriate for earthquake-resistant design.

Codes in many foreign nations have formed in general the provisions of

the SEAOC Code. Others have developed independently using different concepts.

190
Comprehensive listings of world seismic codes have been compiled [2, 3].

II. Status of Design Criteria and Procedures

Production of Earthquake- Resistant Buildings

Structural design is only one element of the processes necessary to

produce a building with adequate earthquake resistance. Each element is

vital; neglect of any one may produce a building with insufficient resistance. The total effort should emcompass these general categories of

el ements

:

• Knowledge of site seismicity hazards. (See review articles by

Donovan and Wiggins et. al.)

• Allocation of required space and architectural requirements in

shaping the building. (See review article by Hillman and Mann)

• Compatibility of architectural layout and structural concept.

(See review article by Hillman and Mann)

• Choice of structural options.

• Structural design procedures and criteria. (Principal emphasis

of this paper, see also review articles by Donovan, Bresler, and

Sha rpe et a 1 .

)

I Compatibility of structural design with non-structural elements.

(See review articles by Hillman and Mann, Ayers and Sun)

• Structural contract documents.

t Selection and organization of the construction team,

t Quality control, supervision and inspection.

• Maintenance after construction

The degree to which each element can be achieved is dependent, in part,

on the importance of the building to the owner or to society, and on the

consequences of failure. Importance of the building will be reflected in the

amount of money available to perform each of the elements. For most buildings, funds needed to perform all currently feasible design procedures are

not available, and approximations in analysis and the sound application of

good judgment by the designer are substituted as alternatives to a comparatively expensive, rigorous analysis. For some buildings, such as nuclear

reactors, each of the elements listed must be performed with the best possible

prof i ci ency

.

Knowledge of Site Seismicity Hazards

The review articles by Donovan and Wiggins et. al . cover this element in

detail. It should be kept in mind that the need for detailed knowledge of

site seismicity may not be required for all building types. In many cases,

design decisions can be made adequately from a cursory examination of site

seismicity. Broad area zoning with appropriate modifications of design

criteria is still a valid design approach.

191
Space Allocation, Archi tectural -Structural Compatibility and Choice of

Structural Options

The early conceptual -desi gn decisions on space allocation and architectural layout are especially important, particularly as they affect the choice

of structural systems. These elements are covered in detail by Hillman and

Mann; they are mentioned only to emphasize their importance in the total effort.

When the structural engineer studies various options for the structural resisting system in a proposed or established architectural layout, cost is usually

the determining factor. It has been demonstrated that in instances where

seismic resistance is of primary concern, particularly if inelastic deformations are anticipated, the choice of a structural system that is only slightly

more expensive will provide a much higher degree of seismic resistance and a substantially reduced damage potential [4, 5]. The design engineer frequently

finds it difficult to justify the added cost, especially when the governing

building code does not consider these potential benefits in its deterministic

format, a format in which loads and membe r- res i s tances are specified - usually

on the basis of past experience and judgment. Current building codes, in an oversimplified manner, attempt to get better structural systems by assigning

different modifying factors to the force function based primarily on the

observed behavior of each system-type during earthquake motions [1].

Structural Design Procedures and Criteria

This paper will discuss structural design procedures and criteria more completely. The influence of design criteria in the total effort of producing earthquake-resistant buildings, however, should be kept in mind. It

is no more important than the other elements.

Structural Compatibility with Non-Structural Elements

The importance of the behavior of a building's non-structural elements

with respect to life, safety, and cost varies with the type of building and

with the function of the non-structural element. Provisions should be made,

for example, that elevators will protect the life of cab occupants, that

exitways will provide for evacuation of the building after major shaking and,

certainly, that facades will not collapse. Most economic losses in earthquakes have involved non-structural elements in and on buildings. In most

cases, the design of the structural frame can assist in providing seismic

resistance for non-structural elements. (Detailed discussion in the review

articles by Hillman and Mann, Ayers and Sun)

Structural Contract Documents

Proper presentation of the design concept on structural drawings and in

specifications is an important element of earthquake-resistant design. Unclear or confusing items in the contract documents lead to errors and mistakes

in the contractor's performance and thus a building with reduced earthquake

resistance. Drawings should be as complete and easy to read and follow as possible. A complex specification calling for unusual materials or unusual

performance by the contractor may result in poorer construction than would a less rigid specification permitting the contractor to act in his customary manner

.

Selection and Organization of the Construction Team

The assembly of an organized construction team is an equally important

element. This implies cooperation among the general contractor, his subcontractors, the architect or prime designer and his consultants. With a

1 92
diligent contractor, quick solution of problems will improve the flow of

work. It has often been noted that if a major item on a job goes awry, many

subsequent errors occur.

Quality Control, Supervision and Inspection

The effort by the construction team to ensure that the work conforms to

the design concept and specifications is an essential element. Most important are the quality control measures set up by the contractor and his subs.

The duties and limitations of all segments of the design team (the architect

usually has the major role in defining these assignments) should be clearly

understood by all parties. Periodic observation by the design team and/or

close inspection by assigned inspectors or building officials should in no way detract from the builder's quality control measures and should serve only

as an independent check to verify the results of the contractor's efforts

[6]. The effectiveness of compliance with the plans, specifications and

building regulations has been historically demonstrated. Also, in general,

the degree of compliance has varied with the effectiveness of the independent

checks on the contractor's quality control.

Maintenance After Construction

Though it may seem obvious, the necessity of maintaining the integrity

of the structural system of a building after completion is frequently overlooked. Removal of a brace member, inappropriate placement of a new opening

in a shear wall, and other alterations to a building are frequently in the

inspection of buildings damaged by earthquakes. Such alterations, when made

without proper engineering considerations, can seriously impair the building's

seismic resistance.

To Summarize: The subject of this paper, structural design procedures and

criteria, is essential, but successful achievement of all the elements outlined is necessary to produce a building capable of adequate earthquake

res i stance

.

Trends in Specifying Design Criteria in Building Codes

Code Design Philosophy

Basic building design philosophy states or implies that little structural damage should occur during moderate intensity earthquake ground motion

but that some damage could occur to other elements in the building. It is

understood that for very high intensity earthquake ground motion some structural damage could occur but that the possibility of structural collapse

should be minimal. Since the San Fernando, California, Earthquake in 1971,

the further requirement has been proposed that buildings housing emergency

services critically needed by the public for post-disaster functions be

designed, detailed, and constructed to remain usable and operable after very

intense ground shaking [7].

Design Factors of Safety

Current design practices uses deterministic analyses - analyses based

on an evaluation of the effects of specified loads (some maximum, some minimum, some mean) on the structural system as compared to a set of specified

working capacities. The relationship between the specified load effect and

the specified resistance contains a factor of safety, which encompasses

provisions for the many uncertainties that are seldom isolated in the design

process. This design procedure was prevalent during the development of

193
seismic provisions when criteria for seismic design were adapted from experience using what has become known as "working stress design methods." One of

the uncertainties contained within the factor of safety was the reliability

of predicting not only the load effects but also the resistance capabilities.

With increasing refinement in determining both sides of the design equation - particularly since the advent of the computer - the actual factor of safety,

in general, has been reduced. This has resulted from "precise" stress analyses on the one hand and more accurate determination of maximum stress capacities on the other, but with no change in the specified design seismic

forces, which were never intended to relate directly to observed maximum

ground motions.

Originally, seismic design criteria were merely design procedures used

to develop a somewhat consistent sizing of elements. Because the design

process did not acknowledge some of the inherent seismic resistance, except

in an implicit manner, seismic resistance depended not so much on the size of

elements but on the detail and arrangement of the component parts and their

fastenings. Some of this implicit design included items as described by John

Blume in his discussion of the Reserve Energy Technique [8]. Much of the

earthquake resistance provided in buildings has not been due to the solution

of the design equation (force-less than-or-equal to-resisting stress) but due

rather to the details of design which were not covered by specific criteria.

This is still true today if the design model does not match the actual building. The taller, fairly regular buildings can be mathematically modeled with

a reasonable degree of accuracy; most buildings, however, have resisting

systems for which realistic models are very difficult to develop. In addition,

the placing of non-structural elements in the building often has a profound

effect on modifying the assumed model.

Current code-specified forces, anticipating working stress design procedures, have not been specifically related to the response generated by an earthquake. The specified forces were determined as the envelope of the

response of a uniform, multi-degree of freedom (MDOF) system. The code

forces were intended to provide elements reasonably sized, consistent with

past practices, to resist earthquakes, and to require the designer to create

the details necessary to provide adequate ductility for the building.

Concurrent with the development of the present code-prescribed minimum

design forces, the codification of load factor design principles in "ultimate

strength design" (USD) in concrete, and in plastic design in steel were being

presented. In some instances, particularly in concrete columns, the "factor

of safety" was effectively and substantially reduced. The concept of load

factor design requires that a factored specified load effect (UQ) shall be

determined to be equal or less than the minimum specified elastically determined resistance (0R). For use in this paper it is assumed that Q and R are specified deterministic load and resistance factors, and that U and 0 are factors to modify Q and R to give the maximum load and the minimum resistance

effects. This concept is the first instance of a commonly used system in

which "factors of safety" are specified. Thus, the specified factor of

safety would be L[. The trend which this concept portends is important in

0 better understanding the ultimate behavior of materials and in designing for

earthquake resistance. Due to the change from working stress, however, a close look should be given to the UQ<0R relationship so that the specified

loads will give the desired "factor of safety."

The safety factor is a much-studied concept, particularly as it applies

to the usual dead and live loads. Simplified probability-based structural

codes have been proposed [9 through 17]. These could be extended to include

the concepts of elastic and inelastic response of buildings to earthquakes

and it is felt that this should be the direction in which the development

of new design criteria should advance. Extensive development work and education of users is needed, however, before general acceptance of the proposals

194
can be expected. The concepts of probability are useful in describing some aspects of the earthquake design problem. For vertical loading, the following curves can be drawn [9]:

These curves represent probability density functions. They are indicators

of the probabilities of occurrence at various levels "x" of load, fgCx), and

of resistance fR(x). The area under each curve is equal to unity or 100%.

The fn(x) curve represents approximately the distribution for dead or live

loads. Usually, fR(x) represents the ultimate strength of the resisting

element. A safety factor has been assigned in several ways. One termed

"central safety factor," is defined (Ro over Qo). If the specified loads are considered as a maximum Qmax (usually dead loads are mean loads) and the

specified resistance of the material is considered a minimum, the specified

factors of safety would be (Rmin over Qmax). In the case of concrete USD,

this latter ratio theoretically is 1, if Qmax = UQo and Rmin = 0Ro. In the case of earthquake loading, the curves are more like the following:

Assuming the design equation of Qmax < Rmin is still desired relationship,

it becomes important to choose the level of Qmax to be used in design. The

"central safety factor (Ro over Qo) has no meaning in this case. The problem

of assigning design criteria and procedures can be stated as the selection of

the appropriate Qmax to be associated with Rmin. The following discussion

summarizes some of the items of consideration in the selection of Qmax and

Rmin for earthquake design.

Ground Motion

Many studies have dealt with the problem of determining the anticipated

ground motion at a site [18, 19, 20, 21]. The subject is discussed in detail

in review articles by Donovan, Algermissen, and Wiggins, et. al . Both the

probability of damage (risk) and the probability of survival have been used

as parameters for design decision [22].

Response Spectra and Modal Analysis

A detailed description of dynamic analyses of buildings is given in

review articles by Newmark and Hall, Sharpe et, al

.

1 95
A response spectrum is a plot of the maximum responses of different

oscillators with varying fundamental periods to a given input motion [23].

The plot can relate acceleration, velocity, or deflection response to the

fundamental period. Response spectra can be developed for many types of

vibrators. The one most frequently used is an elastic single degree of

freedom (SDOF) vibrator with varying amounts of viscous (proportional to

velocity) damping [20, 24, 25]. Similar spectra can be developed for

vibrators responding i nel as ti cal 1 y , for vibrators having other types of

damping and for those having mul ti -degrees of freedom. The inelastic

response most frequently used assumes that members of the frame respond

elastically (stress proportional to strain, that is) to a specified stress

level beyond which strain increases without further increase (or decrease) of

stress. The MDOF system could be one having a constant mass-to-stiffness

ratio, one deforming primarily in flexure, one deforming primarily in shear,

or any specific relationship desired.

SDOF elastic spectra have been most frequently proposed for use in

design. Smoothed spectra are drawn which either envelope the individual

spectrum determined from a number of earthquakes, or a spectrum is chosen

which would give the same response to the average MDOF system. The information necessary to make an accurate choice of the appropriate damping factor

for the building system is not available, particularly if the effects of

interaction between the structure and the foundation media (both rocking and

trans 1 ati onal interaction) are assumed to be an equivalent amount of damping.

As a design procedure, however, an accurate prediction is not always required.

The analysis using the SDOF elastic spectra is accomplished by performing the

following operations:

• Determine the elastic mathematical model that best represents the

building to be built.

• Determine the period (frequency) and the mode shape of the first few

modes of vibration of the assumed model.

• Determine the modal participation factors which are factors describing

the portion of the total mass of the building which is associated with

each mode.

0 Determine the base shear and the distribution of shears throughout

the structure for each mode using the specified SDOF spectral value

with an appropriate choice of damping ratio.

• Combine the responses of the various modes by a suitable means. Combinations most frequently used are the square root of the sum of

the squares or the straight addition of modal effects.

The complex portions of analyses of this type involve the assumption

of the appropriate model and the determination of the mode shapes, although

approximations can be made. The relevancy of the results, however, is only

as good as the validity of the initial assumptions. The placing of non- structural elements often will modify the assumed model and thus completely

alter the building response. Added elements may require multiple analyses

if the effect on the mathematical model is severe enough.

One difficulty presented by an analysis using spectra is that reference

to an actual earthquake event is usually a maximum elastic response. The

time or the number of high amplitude cycles is not specified. Thus, for a given period, the same spectral value could be describing a single cycle of

motion of a small earthquake or the maximum of many 1 a rge- ampl i tude cycles of

a large earthquake. If all response were fully elastic, the number of cycles

would have no significance. Considering the complex structures used for

buildings, however, it is doubtful that true elastic behavior occurs even though this assumption is made for the design model. The number of cycles of

196
significantly high accelerations would make a difference in the extent of

damage a building would undergo should inelastic deformations occur.

As a design method, the analysis by spectral modal analysis can be con- sidered a static load approach as time (an indicator of duration of load and

motion) is not a variable. This is so even though the analysis provides the same maximum response in each mode as would a time-dependent dynamic analysis.

The MDOF spectra currently used as the basis of design are usually considered

as static forces applied to the building. It has converted the dynamic

problem to a solution which uses the same procedures as for other loading

conditions. The dynamic nature of the problem, however, should never be

overlooked by the designer.

It should be remembered that MDOF response spectra, similar to those in

current codes, are based on the assumption of uniform mass-to-stiffness ratios.

Designers who follow the strict wording of the "Code" tend to overlook this.

For a building with non-uniform ratios at various levels, the base shear and

distribution of shear should be modified to an MDOF spectrum suitable to the

particular building.

Time-History Response Analysis

The full dynamic analysis can be accomplished by use of all or a portion

of the motion record of an actual or assumed earthquake [20, 25]. The reliability of predicting the response of a structure is still dependent on the

choice of the appropriate mathematical model, including the damping value.

The earthquake motions in suitably small increments of time are used and the

sequential responses of the assumed model to these motions are determined

based on the equations of motion. Approximate or assumed inelastic as well

as elastic behavior can be modeled. Usually the ground motion used in the

analysis is independently determined at the base of the structure. The

building model may or may not account for the properties of the supporting

foundation material. By use of finite element analysis, the motion at the

base rock can be assigned and the vibrator model composed of the supporting

soils and the structure can be used. This technique can also be used in

three dimensional analysis. The cost of analysis mounts, however, as additional variables are added to the equations of motion. Fully elastic response

is usually used in this type of analysis and the model is somewhat simplified

to reduce the cost of analysis. The relevancy of the results depends on how

well the analytical model describes the actual building. The choice of the

time-history record to be used can be that of an actual record, a predetermined or modified "standard" earthquake, or an earthquake generated by use

of filtered "white noise" techniques to produce a certain result on a standard

vibrator or one to match a chosen SDOF spectrum. Design criteria should not

be devised so that the use of the time-history dynamic approach is denied,

particularly for those buildings with longer fundamental periods. Due to the

uncertainties of model- and earthquake-selection, however, a time-history

analysis does not necessarily provide the engineer with a better design than

that obtained using other methods. The choice of the appropriate model for

shorter period buildings is at the present time very difficult.

Consideration of Inelastic Response in Design

The cost of including specific inelastic motions in response calculations

is usually too great relative to the value of the information obtained. In order to simplify the problem, it can be assumed that inelastic response

follows a bilinear path. One such method is commonly referred to as the

el asto-pl asti c load-deformation curve from which plastic design concepts

could be applied. Even the cost of performing a bilinear analysis usually

is not warranted except for some occupancies. Another approach has been to

perform a fully elastic response analysis with the assumption that maximum

197
deformations will be the same as with el asto-pl asti c response, an assumption

that has been shown in some cases to produce incorrect results [26]. This

approach does not necessarily reveal local instability effects but at least

provides the engineer with probable locations in the structure which should

receive closer scrutiny and gives him valuable information for making design

deci s i ons .

In most design situations, inelastic analyses involving time-history

responses have not been performed. It is assumed that the ductile behavior

of the framing system will adequately support inelastic response. This

ductile response is accomplished by providing adequately sized and spaced

reinforcement in reinforced concrete construction and by appropriate selection of type, shape, and connection in structural steel construction. In smaller buildings, the use of properly detailed wood framing generally has

exhibited adequate ductile response, provided that proper consideration has

been given to connections and control of construction quality. Combinations

of construction methods can also be used and minimum code provisions have

been developed for these items. Additional collateral material that can absorb energy as it deforms also provides inelastic response capacity. This

material should be detailed so that it does not become a hazard. Lacking

"ductile" capability, the elements should be designed to be able to respond

essentially in an elastic manner to design earthquakes.

Limiting Conditions (Limit States) of Design Procedures

In designing for seismic resistance, two limiting conditions (limit

states) should be considered. A lower limit is the design elastic response

of the structural model (elastic limit, Qmax < Rmin). A maximum limit is the

response to an earthquake which is determined to have the maximum intensity

of ground motion critical to the particular structure under consideration.

For this latter case, Rmin in the equation Qmax < Rmin is not necessarily the

ultimate strength. In geographical areas where it is determined that the

potential of high earthquake motion warrants consideration, these two limits

are usually assumed separately. Where the potential for high earthquake

motion is determined to be not great enough for consideration, one limit may

suffice. Methods have been established wherein the probability of damage

(risk) or the probability of survival have been associated with the limit of

elastic response. Where the maximum limit is equal to or closely associated

with the elastic limit, the establishment of the relative risks involved can easily be a parameter that can be introduced into the design equation. The

effectiveness of this approach depends on the accuracy of the prediction of

potential earthquake motions. This is covered in detail in review articles

by Donovan and Algermissen. As this date, however, the state-of-the-art of

prediction of ground motion, particularly near the fault trace, has not produced procedures which have gained general design acceptance.

Another condition to be considered in design is a limitation on deformations. This limit could be based on any loading condition to serve as a guide to minimize pounding between units, as a check on building stability,

as protection of non-structural elements, and for maintaining serviceability

of the building. In order to provide the most visible indication of behavior,

the "maximum limit" ground motion should be used. Lesser ground motion with

appropriate limits, however, could be used if by so doing the analysis is

simplified.

Arbitrary Design Criteria

The use of performance criteria for seismic design would greatly simplify the code language. The application of such performance criteria in

practice, however, could result in great misuse of materials due to the

current lack of basic understanding of the resistance of building components

198
to dynamic loading, particularly response to the higher frequency vibrations

found in earthquake motions and inelastic response. Also, chances would be

greater for misinterpretations by the design profession, the regulating profession, the regulating agencies, and the building contractors. As a result,

some arbitrary limitations have been placed on the use of some materials [1].

In general, these limitations have been to restrict the use of systems to

those areas in which either experience or tests have demonstrated their

applicability. (See review article by Bresler for current status of testing.)

Relaxation of these arbitrary limits should be allowed only as additional substantiating information becomes available.

Considerations for Future Code Criteria

Minimizing Complexity of Design Criteria and Procedures

Future design criteria must be of such a nature that there will be

uniform interpretation and application of the intent by design engineers and

the regulating agencies. Extremely complex formulation will lead to varied

interpretation and varied application. Complex criteria also tend to generate

a greater incidence of error in analyses, which offsets many of the benefits

from such criteria. Any rules (as separate from methods of analysis chosen by

the design engineer) should be easily understood by the practicing engineer.

Such rules should not invalidate the use of more complex design procedures if

deemed appropriate by the design engineer.

Use of Past Experience

Reviewing the overall earthquake experience, it can be said that designs

based on current design procedures have produced buildings with a high level

of seismic resistance for a comparatively low level of cost. Most of the

damage in recent earthquakes has resulted from shortcomings in building layout, design execution, detailing, or construction quality control. Some of

the experience gained by observations of the behavior of buildings, however,

must be considered when establishing new criteria. A few instances of damage

have indicated that the design criteria used were insufficient. Some of this

evidence of insufficient design criteria:

1. All major earthquakes have indicated the poor performance of

unreinforced masonry. The problem of the eradication of existing

unreinforced buildings in earthquake-prone areas has the greatest

potential impact on society. We either eliminate the potential

hazard at tremendous cost or we accept the risk of a great loss

of life. The most effective way to reduce the total hazard problem in earthquakes is to solve the problem of old buildings. See

review article by McClure. Examples: 1971 Sylmar VA Hospital;

1933 Compton, Long Beach; 1952 Kern County.

2. Recent earthquakes have indicated that the axial stresses in

columns within a structure resulting from overturning moments

are determined by the actual stiffnesses and capacities of the

building rather than by the specified design forces acting on an assumed resisting system. This was indicated particularly in the

Caracas Earthquake of 1967 [27].

3. The need to provide ductility and adequate shear capacity to

concrete columns was demonstrated by the San Fernando Earthquake

of 1971 [1 , 28, 29, 30].

4. Also, the San Fernando Earthquake demonstrated the need to have a new philosophy of design with respect to structures required for

post-earthquake recovery.

199
5. The need for a comprehensive review of design levels as related

to buildings with medium fundamental periods (say 0.25 to 1.5 sec.)

was indicated by the Santa Rosa and San Fernando earthquakes [28,

29, 30, 31].

Soil Factors in Design Criteria

Most U.S. building codes do not contain specific soil factors in design

criteria. The differences in the elastic response of structures (on varying

soil structures) has been recognized for some time, but a specific way to

provide for soil effects in a code has not gained general acceptance [21, 32].

This is particularly true when considering the inelastic response of buildings.

Where earthquakes of moderate magnitude are associated with many faults in an area, the determination of the potential shaking and the effects of soil on response becomes very uncertain. Reasonable modifications of the design equation to account for the change in potential shaking with varying soil conditions should be considered.

III. Status of Applied Research

Reconciliation of Data from Earthquake Records

During the 1971 San Fernando Earthquake, more than 200 usable accelerograph records were obtained. Many of these were obtained in taller buildings

because the Los Angeles City Building Code requires three instruments in

high-rise buildings - one at the roof, one at the lowest level , and one at an intermediate level. Most of the instrumented buildings had a response to the

earthquake shaking which was primarily elastic. Two, at least, indicated

definite structural inelastic response. Analyses have been made on a number

of these instrumented buildings [23, 27, 28, 29]. The general conclusion

from these studies is that buildings having long fundamental periods (say 1.0 sec. or more) can be reasonably modeled for determining response to earthquake motions. The amount of damping required for reconci 1 ati on varied, but

a reasonable value could be selected for use in design. Whether damping

values should be itemized in a building code is a matter for consideration.

Research on Materials of Construction

A detailed report on the status of the resistance of materials of con- struction is given in the review article by Bresler. A few items of overall

interest to the development of design procedures will be discussed here.

Most testing of materials for earthquake resistance has used either

static loading or loading at a very slow rate. Also, most testing has been

done without reversal of loading. The tests do give a reasonable representation of the behavior of materials in the case of taller buildings, however,

where the fundamental period is long (as mentioned before, on the order of

1.0 or more). For shorter period buildings, the relationship between static

testing and dynamic response may be quite different. For instance, in blast

design, the effect has been accounted for by an increase in the compressive

capability of concrete. The effect of fairly fast loading on the buckling

characteristics of steel shapes is not well defined nor has the effect on shearing strength of concrete been adequately demonstrated.

Until further research provides data necessary to answer the questions

regarding short-period loading, reasonable assumptions must be made based on the actual experience of past earthquakes and on the results of the available

static tests.

200
Each type of construction material responds to earthquake shaking

differently. This is particularly true when inelastic response is considered.

Thus, for concrete the problem of "non-ductile" compressive and shear failures

is of major concern. Local, lateral, torsional, or shear buckling is the

special concern on the inelastic behavior of structural steel elements. The

special concern in cold-rolled sheet steel diaphragms is the development of

adequate fasteners of individual panel units and the development of diaphragm

chords and struts. Similarly, for wood diaphragms, the principle concern is

the development of struts and chords and, as indicated by the 1971 San

Fernando Earthquake, the development of adequate lateral support of adjoining

walls. The special concern for masonry and pre-cast construction is the

development of adequate connection between individual elements. Joinery is

of concern for each material but the development of adequate joint details is

distinct for each material. The properties and details of construction using

each material require that separate studies be made to determine the special

requirements to establish design criteria. Until this is done, consistent use of limit strength procedures cannot be performed by the design engineer.

Research on the Effect of Foundation Materials

In the past, some research has been done on reconciling the effect of

foundation materials on building response. Much more needs to be done. The

data from the San Fernando Earthquake should be analyzed to determine if

current theories are applicable. There is some indication they are not.

Research on the Design Earthquake

It is now possible to collect data currently available to arrive at

design criteria applicable to buildings with longer fundamental periods. The

development should be done using probabilistic concepts which could be modified to deterministic terms for code use. Additional basic research is

required before such an approach can be undertaken on buildings having short

periods, and until such research is completed, criteria for these buildings

should be based on current experience in the damage surveys of previous

earthquakes. The interface between these two approaches should be such that

no large discontinuity exists between the products of the two design approaches. This task is one that should be started as soon as possible so that

currently available expertise can be made useful.

IV. Research Priorities

Current Goals and Priorities in Applied Research

In the past, the criteria for earthquake design in the United States

have been established primarily by volunteer committee effort by members of

the four associations comprising the Structural Engineers Association of

California. The task of assembling the criteria into legal code language and

presenting it in a form suitable for practical application by design engineers

has become too great for volunteer effort alone, especially considering the

speed of change being asked by political and administrative bodies. A new approach has been launched, therefore, with the intent of bringing together

the expertise of the practicing engineer and the data from the specialists on seismicity, risk and dynamics. The new approach is through an SEAOC-sponsored

corporation, Applied Technology Council, to organize research and assemble

data for new design criteria under the guidance and supervision of practicing

engineers so that the criteria will be workable and practical.

A broad outline for this work is contained in a position paper prepared

by the Seismology Committee of SEAOC [36]. This outline provides the back201
ground for a list of items to be followed in developing revised seismic

design provisions to reflect current knowledge and experience. This work

should be the first priority for the tasks ahead.

1. The attainable goals of the seismic code should be clearly defined in

the areas of:

• Structural damage,

t Non -s tructu ra 1 damage.

• Post-earthquake functions,

t Human risk.

Comment : Current commentaries on seismic codes do not

line the various damage levels expected to be obtained

the provisions even though the basic design philosophy

ed. After arriving at the decisions on later items in

specific areas of concern should be delineated so that

properly assess the risks involved.

2. Basic provisions of the seismic code should contain:

t Equivalent static forces for most structures.

• Dynamic and inelastic analyses for others which would be

a. Required in certain cases.

b. Optional for all structures.

• Simple factors for low buildings.

Comment : The first and third items are the design approaches currently

in use in seismic codes. The experience to date has indicated that when

designing structures which are applicable, the concepts need not change.

The equivalent static force approach (MDOF spectra) currently in use for uniform buildings is recommended with some possible modification in

quantities. For one- and two-story buildings constructed with a reasonable amount of inherent damping, the criteria should remain as a specified per cent of gravity. This could vary, however, depending on site

location and conditions and on importance factors.

The new added item is codification of the requirements for dynamic

analyses. Specific inelastic analyses should be required only on structures for which the probability of failure is to be reduced to a minimum. The types of structures to be included in this category have

been the subject of frequent discussions. Usually the only types con- sidered are those housing nuclear reactors. Emergency communication

centers might also be included. However, consideration for inelastic

behavior should be provided in all classes of structures.

3. Seismic codes should include provisions for determining:

a. Basic realistic levels of ground motion to represent a design

earthquake at a site of average exposure having no unusual soil

conditions.

b. Necessary levels of structural systems damping, ductility and

stability to survive this ground motion adequately including

techniques for handling the P-delta problem.

Comment : The choice of the appropriate levels to assign to both sides

of the design equation LIQ<0R and the manner of presentation should be

made with consideration for all elements of design, including the timedependent problems of overturning and building stability. Specific

numbers for the levels will follow once the basic concept of design is

specifically outby application of

has been expressthis list, these

all concerned can

202
chosen based on experience to date.

Tolerable levels of cracking for brittle materials should be established

Comment : This level should be reviewed so that criteria for drift

limitations may be formulated and so that a better understanding of

cost risks may be established in the code.

Factors for determination of equivalent static force design should be

established.

a. Determine C factor to apply to a structure of minimal ductility

for the realistic response to ground motions outlined in 3. a.

b. Establish K reduction factors based on the items in 3.b.

c. Establish S factors for site soil effects.

d. Establish importance factors I based on human risk and postearthquake use of emergency facilities (deformation or force

factor) . e. Provide for modification of the zone factor (Z) for force function

and details when data on site exposure become available. Review

zone factors for areas other than California.

Commen t : The specific factors relating to the equivalent static forces

currently in use should be reviewed. There is some indication that in

the medium period range (0.25 to 1.5) the relative values of C are not

consistent with those in the short and long period ranges. An examination of the currently used force modifying factor K should be undertaken

to see if the definitions for the various categories should be changed

and to determine if current values are appropriate. The data are available to make a decision on whether or not soil factors should be

included in the code or whether they should be left to individual

determination for each site. The inclusion of importance factors at

this time should be restricted to buildings housing facilities necessary

for post-earthquake use and to certain classes of buildings in which

probability of failure should be minimized. The current zone factors

for broad areas of varying seismicity should be reviewed (i.e. 1, 0.5,

0.25) particularly as they relate to the choice of details of construction.

Realistic deterministic ultimate design stresses and load and resistance

factors should be established for all materials consistent with the

design factors above. Uniformly consistent load factors should be established for various loading conditions which are common for all material

Variability factors should be established for different materials, com- binations of materials and types of construction. Arbitrary limits and

details necessary to accomplish these factors also should be established

Comment: See Comment on 3.

Conventional one-story light frame requirements should be redrafted

consistent with requirements for designed structures.

Comment : The design criteria for conventional one-story light frame

construction has been specified by standard minimum details rather than

by stress design. This method should be retained but a review of the

commonly used arbitrary construction details should be undertaken to

make the provisions consistent with the level of safety chosen for other

cons tructi on

.

203
8. The classes of structures requiring dynamic analyses should be

establ i shed

.

Comment : The choice of those structures to be named in the code to

have a specified kind of dynamic analysis should be carefully considered

so that the cost of analysis will not be out of line with the risks

involved. Currently under consideration by the ICBO is a provision as

f ol 1 ows : "Buildings or structures which have highly irregular shapes,

large differences in lateral resistance or stiffness between

different stories or other unusual structural features

affecting seismic response shall be designed for forces

which their dynamic properties induce."

Also, consideration should be given to requiring modal analysis for

buildings having fundamental periods in excess of 1.0 to 1.5 seconds.

9. Specific criteria should be established to be used for dynamic analyses

and designs consistent with the criteria used in determining the equivalent static force criteria above but with some reduction in conservatism so as to encourage dynamic design. Both service level and inelastic

criteria should be established.

Comment: See Comment on 3.

10. Drift limitations should be established consistent with the realistic

response to strong earthquakes. Consider tolerable limits of non-structural materials as well as structural systems, materials and stability.

Establish building separation limitations.

Commen t : Compatibility with architectural, mechanical, and electrical

systems should be considered for establishing limits.

11. Where required, criteria should be established to cover the vertical

acceleration problem.

Comment : Existing data on the concurrent behavior of vertical and

lateral motions should be reviewed. Provisions should be incorporated

. in the design code to account for this behavior. This can be done by

appropriately modified load factors. Basic study should continue on this subject.

12. Shear wall - frame interaction provisions should be established.

Commen

t

: Special shear wall - frame interaction provisions are required

so that the difference in ductility between the two systems may be

accounted for in elastic analyses.

The remaining items are independent subjects but are necessary for completing

a worki ng code

.

13. Criteria should be established for the repair of earthquake-damaged

buildings.

Comment : Even though very little information is currently available on the efficacy of repair procedures for buildings damaged by earthquakes.

204
specific provisions for repair should be included so that recovery in

a damaged area can be facilitated.

14. Criteria should be established for rehabilitation and upgrading of

existing buildings.

Comment : As mentioned in Section 1 of this paper, the practical solution for a corrective program would go further than any other provision

toward reducing the earthquake hazard relative to loss of life.

15. Seismic design requirements for mechanical and electrical equipment

should be established.

Comment : The requirements for these items should not only be the

assignment of a lateral force of x per cent of gravity; for some pieces of equipment a more sophisticated analysis will be required.

Assignment of the responsibility for design should also be determined.

16. Minimum requirements for quality control should be established.

Comment : An outline for effective quality control should be given.

The varying abilities of communities to perform the checks of the

contractor's quality control should be a factor in the level of design

just as the ability of a community to combat fires is considered in

insurance rates.

17. A commentary should be written and it should be complete and instructive

for those not experienced in seismic design.

Comment : The use of a building code is dependent on a common understanding of the intent of the code provisions. A detailed commentary

explaining the intent of the various provisions is necessary.

The foregoing list should remain flexible for revision as the work proceeds.

V. Basic Research Priorities

While the state-of-the-art criteria are being developed, basic research

which could contribute to a better understanding of seismic motions and

structural response should proceed. Research should be designed to produce

information on the following items:

1. Shear walls of all materials to determine

a. Capacity and ductility, particularly in resisting high

frequency motions.

b. Tolerable strain and/or cracking.

2. Ultimate column capacities and limitations for all materials.

3. Frame connection capacities and limitations for all materials.

4. The characteristics of elastic and inelastic frame instability to

earthquake moti ons .

5. Inelastic capability of braced systems.

205
6. The capacity and limitations of connection details other than for

frame members (all materials)

a. Tying together of building elements.

b. Requirements for details on drawings.

7. Damping and energy absorbing characteristics of various materials

and assemblies.

8. Efficacy of modeling techniques for various framing systems and

materials at various fundamental frequencies.

9. Vertical acceleration effects on buildings and structures.

10. Amount of reserve energy capacity in buildings of various categories.

11. The effect of three dimensional seismic wave motions on large

building configurations.

12. Soil effects on ground motion at site.

13. Soi 1 -structure interaction.

14. Soil damping and strength characteristics consistent with the

design equation.

15. An extension of the nework of strong motion acce 1 e rographs should

be implemented. Smaller buildings than those that are currently

covered by laws such as exist in Los Angeles City should be

instrumented. Servicing of the instruments by the National Oceanic

and Atmospheric Administration should be continued. Uniform

criteria for instrument location should be developed so that in-

; formation can be more easily interrelated. Means should be established for rapid data reduction and analysis of earthquake records

when they become available.

16. Development of probability-based code provisions.

17. Determination of feasible procedures for upgrading existing

hazardous buildings.

18. Implementation of the construction of a large, three-component

earthquake simulator [37].

VI . References

1. Seismology Committee, Structural Engineers Association of California,

"Recommended Lateral Force Requirements and Commentary," 1968 and

Appendix F, 1971 .

2. International Association of Earthquake Engineering, "Earthquake

Resistant Regulations - A World List, 1966." Gakujutsu Sunken Fukyu-Kai

Oh-Okayama, Meguroku, Tokyo.

3. Hollis, E. P., "Bibliography of Earthquake Engineering," Earthquake

Engineering Research Institute, 1971, Sec. 6.1.

4. "Report of the Task Force on Earthquake Hazard Reduction," Executive

Office of the President, Office of Science and Technology, September

1970.

206
5. Steinbrugge, K. V., "Federal Policy and the Hazard of Earthquake,"

Proceedings, Structural Engineers Association of California, 1970.

6. Symposium on Responsibility for Inspection, American Concrete Institute

Committees 311 and 348. Journal of the American Concrete Institute,

June 1972.

7. "Report of the Los Angeles County Earthquake Commission, San Fernando

Earthquake, February 9, 1971," November 1971.

8. Blume, J. A., "A Reserve Energy Technique for the Eartiiquake Design and

Rating of Structures in the Inelastic Range," Proceedings of Second

World Conference on Earthquake Engineering, 1960.

9. Freudenthal , A. M. et al . , "The Analysis of Structural Safety," Journal

of the Structural Division, ASCE, February 1966.

10. Ang, A. H. -S, and Amin, M., "Reliability of Structures and Structural

Systems," Journal of the Engineering Mechanics Division, Proceedings

ASCE, April 1968.

11. Benjamin, J. R. , "Probabilistic Structural Analysis and Design," Journal

of the Structural Division, Proceedings ASCE, July 1968.

12. Ang, A. H. -S, and Amin, M . , "Safety Factors and Probability in Structural Design," Journal of the Structural Division, Proceedings ASCE,

July 1969.

13. Benjamin, J. R. and Cornell, C. A., "Probability, Statistics and

Decision for Civil Engineers," McGraw-Hill, 1970.

14. Shah, H. C, "The Rational Probabilistic Code Format," Journal of the

American Concrete Institute, September 1969.

15. Sexsmith, R. G. and Nelson, M. F., "Limitations in Application of Probabilistic Concepts," Journal of the American Concrete Institute,

October 1959.

16. Benjamin, J. R. and Lind, H. C, "A Probabilistic Basis for a Deterministic Code," Journal of the American Concrete Institute, November 1969.

17. Cornell, C. A. "A Probability-Based Structural Code," Journal of the

American Concrete Institute, December 1969.

18. Cornell, C. A., "Engineering Risk Analysis," Bull, Sei smol ogi cal Society

of America, (SSA), October 1968.

19. Benjamin, J. R., "Probabilistic Models for Seismic Force Design," ASCE

National Convention, 1967.

20. Wiegel , R. L. et al . , "Earthquake Engineering," Prentice-Hall, Inc.,

1 970.

21. Lecture Notes "Earthquake Resistant Design of Engineering Structures,"

University of California, June 1972.

22. Wiggins, J. H. and Moran, D. F., "Earthquake Safety in the City of Long

Beach Based on the Concept of Balanced Risk," Report to Long Beach,

California, May 28, 1970.

23. Housner, G. W., Martel, R. R., Alford, J. L., "Spectrum Analysis of

Strong Motion Earthquakes," Bulletin of S.S.A., April 1953.

207
24. Blume, J. A., Newmark, N. M., Corning, L. H., "Design of Multistory

Reinforced Concrete Buildings for Earthquake Motions," Portland

Cement Association, 1961.

25. Newmark, N. M. & Rosenbluth, E., "Fundamentals of Earthquake Engineering," Prentice-Hall, Inc., 1971.

26. Goel , S. C, "Response of Multistory Steel Frames to Earthquake Forces,"

AISC Bulletin No. 12, November 1968.

27. Hanson, R. D. and Degenkolb, H. J., "The Venezuela Earthquake, July 29,

1967," AISI 1969.

28. Lew, H. S., Leyendecker, E. V., Dikkers, R. D., "Engineering Aspects of

the 1971 San Fernando Earthquake," N.B.S. Building Science Series 40,

December 1971.

29. Jennings, P. C. et al . , "Engineering Features of the San Fernando

Earthquake, February 9, 1971," EERL 71-02, June 1971.

30. NOAA/EERI, "Report on San Fernando Earthquake of 1971." In process of

publ i cati on

.

31. Steinbrugge, K. V., Cloud, W. K. , Scott, N. H., "The Santa Rosa,

California, Earthquake of October 1, 1969," ESSA, 1970.

32. Hollis, E. P., "Bibliography of Earthquake Engineering," Earthquake

Engineering Research Institute, 1971. Section 5.3

33. Martin, A. C. and Associates, "Post Earthquake Analysis," Structural

Engineers Association of Southern California, January 1972.

34. Muto , K. , "Strong Motion Records and Simulation Analysis of KII Building

in San Fernando Earthquake," Muto Institute of Structural Mechanics,

September 1971 .

35. Gates, W. E., "Seismic Study of the Certified Life Building," Report

to Portland Cement Association, 1972.

36. Seismology Committee, Structural Engineering Association of California,

"Position Paper On Implementation of October 1971 Report of SEAOC Ad

Hoc Committee on Direction Study - Seismic and Modifications to Seismic

Design Codes," May 1972.

37. Penzien, J., Bouwkamp, J. G., Clough, R. W., Rea , D., "Feasibility

Study Large Scale Earthquake Simulator Facility," EERC 67-1, University

of California, 1967.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards, August 2 8 -S eptember 1,

1972, Boulder, Colorado (Issued February 1973).

208
PROCEDURES AND CRITERIA FOR EARTHQUAKE RESISTANT DESIGN

by

N. M. Newmark and W. J. Hall

Urbana , Illinois

I . Introducti on

When a building or other structure is subjected to earthquake motion,

its base or support moves with the ground. Since this motion is relatively

rapid, it causes stresses and deformations throughout the structure. If we neglect temporarily the interaction between the base of the structure and

its foundation, when the structure is quite rigid, its motion is nearly the same as that of the ground and the dynamic forces acting on it are very

nearly equal to those associated with the ground accelerations applied to

the structure as a rigid body. If the structure is quite flexible, large

relative motions or strains can be induced in the structure because of the

differential movements between the supports and the masses of the structure.

In order to survive the earthquake motions the structure must be either

strong enough or ductile enough to resist the forces generated by these

deformations; the combination of the required strength and ductility is a function of the stiffness or flexibility of the structure.

Seismic effects on a structure, component or element, depend not only

on the earthquake motion but also on the properties of the structure, com- ponent or element itself. Among these properties, the most important are the energy absorption within it or at interfaces between the item under

consideration and its support, either due to damping or inelastic behavior,

its period of vibration, and its strength or resistance.

It is the purpose of this paper to describe the general nature of the

principles upon which earthquake resistant design is based, and to consider

the development of design procedures for the design of structures, facilities

or components.

Examples of structures that did not have sufficient strength and

ductility to resist the earthquakes to which they were subjected are well

known. Failures occurred in the columns and frames of buildings in Caracas,

for example, when inadequate strength and energy absorbing capacity were available for the earthquake of 1967. Other failures in earthquakes are clearly due to lack of adequate support details, or lack of adequate continuity between individual elements.

Empliasis is placed herein on design as contrasted with analysis, and

essentially on preliminary design or the selection of the general outline,

type of framing, and first estimate of requirements. This choice of emphasis

is made because methods suitable for such purposes generally can assure adequate performance and serve as a check on designs made by more sophisticated methods

.

The general concepts presented in this paper have been adapted from

those given in References [1], [2], [3], and [4].

The design of a structure, as either a complete system in itself, or as part of the system of which the structure is only a component, can be a highly complex matter involving a number of input data of various types and

a host of special requirements. Once the structure has been dimensioned,

that is, laid out in plan and the size and strength of its various elements

selected, then the analysis of the structure for given conditions of loading

209
and foundation motion can be made by relatively well understood methods,

even though the analysis can be a tedious and lengthy one for a complex

system. However, unless the designer uses a so-called "direct design" procedure, he is faced with a problem of the preliminary selection of the

structural layout, framing, element strength, and the like, before he has

a structure which he can analyze. Even with direct design procedures, for

important structures he will want to have some handy approximation that can

be used for his preliminary studies.

The steps which the designer must take are generally as follows:

(1) Select the earthquake hazard.

(2) Select the safety factor, or the allowable limits of deformation,

or the allowable probability of damage or failure. This may

depend on step ( 3 ) .

(3) Select the type or layout of the structure, and estimate its

dynamic (and static) parameters. These include a) dynamic

resistance, b) natural frequency or period of vibration, c) damping

or energy absorption, d) ductility that can be counted on before

failure. These may be assigned in a direct-design procedure, or are subject to successive revision in more traditional procedures.

(4) Verify the adequacy of the structure selected, and make any

necessary changes in strength or other parameters, or in the

complete layout or plan.

(5) Repeat steps (3) and (4) until a satisfactory design is achieved.

(6) Make a more accurate analysis of the final design, and make further

changes that may be necessary. If these are not minor, steps (3)

to (5) may need repeating. In some cases revisions in steps (1)

and (2) may be desirable. In other cases an upper bound direct

procedure may be used involving essentially only steps (1), (2),

and (3). Most so-called "static design codes" are intended to be

of this type.

II. Earthquake Hazard

Earthquakes are relatively rare occurrences, but in many regions of the

world one can count on a high probability of at least a small earthquake

occurring once in the lifetime of a building. However, the stronger or more intense the earthquake the smaller is the probability of its occurrence. An

earthquake that has a relatively high probability of occurrence is appropriately considered as a loading for which the design must provide in such a way

that the cost of the minor repairs required is not excessive. Major strengthening of a structure to resist intense forces is expensive, and the cost of

such design provisions must be weighed against the possible cost of repairs

in order to design whether the additional design strength or ductility is

economically justified.

It is generally agreed that structural collapse of such a nature that

it might endanger a great many lives should be prevented by the design, even

for the maximum credible earthquake. But it would be unreasonable and un- economical to provide for resistance to an extreme earthquake with the same factor of safety or margin of safety that one normally uses. The selection

of the factor of safety for the maximum credible earthquake is in part

dependent on the nature and importance of the structure, and on the consequences should the structure fail.

Unfortunately the earthquake hazard for which designs should be made is

210
subject to a high degree of uncertainty. In only a few areas of the country

are there relatively long periods of observation of strong earthquake motions.

By correlating the available strong motion records with the more common records available from the sensitive recording instruments used by seismologists, and by use of qualitative reports of the effects of earthquakes

where motion records are not available, some measure can be obtained of the

maximum intensities which have occurred in various geological regions, and

predictions can be made of those which might occur in the future. In other

regions of the country, where records are scarcer, estimates of a similar

nature can be inferred but are much more uncertain.

However, the maximum historical earthquake determined by such a procedure is not a proper measure of the possible intensity of an earthquake which

might occur in the future nor of the earthquake for which the design should

provide. At some sites, maximum or extreme earthquakes might never have

occurred in the past; it is almost certain that they did not occur within the

period of recorded history.

In order to specify adequately the earthquake intensity for either the

historical or the extreme earthquake, one must do more than determine the

possible or probable acceleration of the ground. The character of the

earthquake motions must also be described in a way that is representative

of the geologic conditions, taking into account the local soil conditions

including overburden depths and characteristics, presence of water, depth

to basement rock, and the like. A better measure of the free-field earthquake motions is a description which includes not only the maximum ground

acceleration, but also the maximum ground velocity and the maximum ground

displacement, with some measure of the number of pulses or the duration of

the strong motions that should be considered. All of these quantities are dependent on the geologic and soil characteristics, and are in part dependent

on the soi 1 -structure interaction of the structure supported on the soil or rock

.

Earthquakes in different parts of the world on different types of

foundations and rock or soil strata have greatly different characteristics.

The Niigata Earthquake of 1964 was characterized by the phenomenon of liquefaction of the soil and foundation failures causing tilt and overturning of

a number of buildings that otherwise would not have been badly damaged. The

earthquakes in Mexico City are affected by the natural frequency of the huge

bowl of soft jellylike soil which underlies most of the central part of the

city, and which emphasizes and amplifies motions in the range of periods of

vibration of 2 to 2.5 sec. and diminishes those of very low period, of less

than 0.5 sec.

The low buildings, the old churches and the cathedral in Mexico, many

of which have been in existence several hundred years during which time

Mexico has been subjected to serious shaking from earthquakes, generally

have not been severely damaged by earthquakes, but modern tall buildings

were seriously damaged, especially in the earthquake of 1957. Nevertheless,

one building, construction of which was completed during the early 1950's,

the Latino Americana Tower, survived this earthquake and, subsequently, the

several slightly less intense earthquakes of the past decade with no damage,

not even cracking of window panes. This is due to the fact that the expected

nature of the earthquake motions was taken into account in its design. The

building is of interest because it was the first building which was designed

in accordance with modern analytical methods that was actually subjected to

an earthquake approaching the intensity of the earthquake for which it was

des i gned

.

With the increased knowledge of the characteristics of earthquakes from

the records obtained of strong motions in earthquakes in various parts of

the world, we now have the basis for a much more detailed description of the

type and intensity of earthquake motions that should be considered for design

of structures of various types in various regions, taking into account the

211
geologic conditions as well as the foundation materials under the buildings.

III. General Design Concepts

General Principles

The designer's freedom of choice in selecting methods of resisting

earthquake motions is restricted by the necessity that he comply with the

architectural form selected for the building. If the form follows the

function, the constraints are generally minimal. However, it is not neces- sarily true that an efficient earthquake resisting capability can be put into

any arbitrary form of envelope for the structure. The designer must, therefore, have latitude in his selection of the resisting elements of the structure. He may choose a flexural framework; or a structure having resistance

primarily in the outer walls as a monocoque assembly; or a structure strengthened by shear walls or by bracing; or a structure with a resisting core from

which the lower parts of the building are hung; or various modifications and

combinations of these. The methods of achieving strength and ductility in

these various forms are necessarily different and the design criteria have to

take this into account.

The permissible level of response of a structure, element or component,

must be associated with a loading criteria. The response criteria should

properly be dependent on the type of structure, the relative cost of repairs

for minor damage, and the hazard in terms of possible loss of life should the

item fail or reach extreme deformation limits. The seismic resistance of an element is a function primarily of its yield strength, its natural frequency

of vibration, its damping and energy absorption in the elastic range, and its

ductility and energy absorption capacity in the range before unacceptable

damage occurs.

Dynamic Resistance

Detailed descriptions of the response of simple elastic systems, or more complex structure and elements, subjected to dynamic loading and

especially to seismic loading, are given in References [2], [3], and [4].

In general, it can be shown that the response of a simple damped oscillator

to a dynamic motion of its base can be represented graphically in a simple

fashion by a logarithmic plot as shown in Figure 1. In this figure, there

are shown on the one plot, using four logarithmic scales, the following three

quantities

:

D = Maximum relative displacement between the mass of the oscillator

and its base

V = Maximum pseudo relative velocity = toD

A = Maximum pseudo acceleration of the mass of the oscillator = ^wD

In these relations, co is the circular natural frequency of the oscillator.

The effective maximum ground motions for the earthquake disturbance for

which Fig. 1 is drawn are maximum ground displacement d^i = 10 in., maximum

ground velocity Vm = 15 in. per sec, maximum ground acceleration aTM = 0.3g,

where g is the acceleration of gravity. The curve shown is a smooth curve rather than the actual jagged curve that one obtains from a precise calculation. The symbols 1, 2 and 3 on the curve represent oscillators, item 1 having a frequency of 20 cps , item 2 of 2.5 cps , and item 3 of 0.25 cps. It can be seen that for item 1 the maximum relative displacement is extremely

small, but for item 3 it is quite large. On the other hand, the pseudo

acceleration for item 3 is relatively small compared with that for item 2.

The pseudo relative velocities for items 2 and 3 are substantially larger

21 2
0.1 0.2 0.5 I 2 5 10 20 50 Frequency, cps

FIG. I RESPONSE SPECTRUM FOR TYPICAL EARTHQUAKE

21 3
than that for i tern 1 .

The advantage of using the tripartite logarithmic plot, with frequency

plotted also logarithmically, is that one curve can be drawn to represent

the three quantities D, V and A. The pseudo relative velocity is nearly the same as the maximum relative velocity for higher frequencies, but differs

substantially for very low frequencies. It is, however, a measure of the

energy absorbed in the spring. The maximum energy in the spring, neglecting

that involved in the damper of the oscillator, is MV^/Z, where M is the mass

of the osci 1 1 ator

.

The pseudo acceleration is practically the same as the maximum acceleration, and the quantity MA is precisely the maximum force in the spring.

Therefore, the pseudo acceleration is exactly the same as the maximum

acceleration when there is no damping.

In the discussion and figures which follow, the terms "velocity" will

be used for V and "acceleration" for A without the explanatory words maximum,

pseudo, relative or absolute.

There are many strong motion earthquake records available. One that

has been used for a number of years is that for the El Centre earthquake of

Hay 18, 1940. The response spectra computed for the earthquake for several

different amounts of damping are shown in Figure 2. The oscillatory nature

of the response spectra, especially for low amounts of damping, is typical

of the nature of response spectra for earthquake motions in general. A re plot of Fig. 2 is given in Fig. 3 in a dimensionless form where the scales

are given in terms of the maximum ground motion components. In this figure,

the ground displacement is given by the symbol y, and the subscript m designates a maximum value. Dots over the y indicate differentiation with

respect to time.

It can be seen from Fig. 3 that for relatively low frequencies, below

something of the order of about 0.05 cps , the maximum displacement response

D is practically equal to the maximum ground displacement. For intermediate

frequencies, however, greater than about 0.1 cps, up to about 0.3 cps, there

is an amplified displacement response, with amplification factors running up

to about three or more for low values of the damping factor 3-

For high frequencies, over about 20 to 30 cps or so, the maximum

acceleration is practically equal to the maximum ground acceleration. However, for frequencies below about 6 cps, ranging down to about 2 cps, there

is nearly a constant amplification of acceleration, with the higher amplification corresponding to the lower values of damping. In the intermediate

range between about 0.3 to 2 cps, there is nearly a constant velocity response,

with an amplification over the maximum ground velocity. The amplifications

also are greater for the smaller values of the damping factor.

The results shown in Fig. 3 are typical for other inputs, either for

other earthquake motions or for simple types of dynamic motion in general.

The data from which Fig. 3 was drawn, as well as other similar figures, are taken from Reference [2].

Natural Frequency

The dynamic response of a structure is a function, among other things,

of its natural frequencies of vibration in its various modes. Natural

frequencies can be computed from the mass and stiffness distributions of the

structure but such calculations involve an idealization of the structure for

the purpose of the analysis. The influence of nonstructural components on natural frequencies can be of particular importance. Also the natural

frequencies may be affected to a large degree by the foundation-structure

214
100

50

a> 20

2

- \

X X

/Vl Dam

^%

ping Fac

.J

\ /

or, %\

M

>

/ o y

s\ / <) \ /

( < \

0.05 0.1 0.2 0,5 I 2 5 10 20 50 Frequency, cps

FIG. 2 RESPONSE SPECTRA, EL CENTRO EARTHQUAKE, MAY 18, 1940,

NORTH -SOUTH DIRECTION

215
216
interaction.

Design specifications which involve natural frequencies have the disadvantage that the structure must be designed, at least in a preliminary way,

before the frequencies can be determined, or else the frequencies must be

estimated from factors involving judgment and overall dimensions. Hence,

such methods may involve relatively large errors in the response or else the

method of design must be one of continuing approximations and revisions.

Dam pi ng

Energy absorption in a structure arises in various ways including

damping or energy absorption of various types within the structure itself,

friction or viscous damping, or other types of damping in the structure as well as in the parts of the structure interfering with each other or moving

against one another. These can all be generally approximated by use of a damping coefficient. The damping is a function of the intensity of motion

and of the stress levels induced within the structural components, and it is

highly dependent on the makeup of the structure and the energy absorption

mechanisms within it and at its interfaces with the foundation or with other

structures. The importance of damping is indicated by the fact that the

dynamic response of a structure in an earthquake may be affected to as great

a degree by damping as by almost any other parameter. This is especially

true in those instances when long sustained nearly harmonic motions are involved. It is because of this reason that the greatest difficulties are found with design specifications other than of the performance type in which

the design forces do not properly reflect the differences in damping asso- ciated with different materials, different types of framing, and different

levels of allowable deformation and stress.

Inelastic Behavior and Ductility

Let us now consider the situation in which the simple oscillator has a spring which can deform i nel asti cal ly during the response. The simple

res i stance -di spl acement relationship for the spring is shown by the light

line in Fig. 4, where the yield point is indicated, with a curved relationship

showing a rise to a maximum resistance and then a decay to a point of maximum

useful limit or failure at a displacement um- An equivalent el asto-pl asti

c

resistance curve is shown by the heavy line in the i'igure, rising on a straight

line to a point where the yield displacement is Uy and the resistance ry , and

then extending without appreciable increase in resistance to the maximum

displacement Uf^. The effective resistance curve is drawn so as to have the same area between the origin and Uy as the actual curve, and again the same area to the maximum displacement point. The ductility factor y is defined as the ratio between the maximum permissible or useful displacement to the yield

displacement, for the effective curve.

It is convenient to use an el asto-pl asti c resistance-displacement

relation because one can draw response spectra for such a relation in

generally the same way as the spectra were drawn for elastic conditions in

Fig. 2 and 3. In Fig. 5 there are shown acceleration spectra for elastoplastic systems having 1% of critical damping for the El Centre 1940 earthquake. Here, the symbol Dy represents the elastic component of the response

displacement, but is not tne total displacement. Hence, the curves also give

the elastic component of maximum displacement as well as the maximum acceleration, A, but they do not give the proper value of maximum velocity. This is

designated by the use of the symbol V' for the pseudo velocity drawn in the

figure. The figure is drawn for ductility factors ranging from 1 to 10. It

is typical of other acceleration spectra for el asto-pl asti c systems, as indicated by the acceleration spectra shown in Fig. 6 for the step displacement pulse sketched in the figure.

217
Actual

o

c

o

tf> 9

^Actual Yield X*

\Point K /^ Effective ^

/

/

/

Displacement

FIG. 4 RESISTANCE - DISPLACEMENT RELATIONSHIP

218
WW _ ui, O

A4!0O|eA-opn9Sd " A

219
^4po|9A punojQ tJunmjXD^

AipoiaA-opnasd

UJ

220
Figure 6 is drawn for a step displacement pulse corresponding to the

two triangular pulses of acceleration shown, where the total length of time

required to reach the maximum ground or base displacement is 1 second. The

frequency scale shown in Fig. 6 will be changed for any other length of time,

t, to reach the maximum displacement by dividing the frequencies f by t. In other words, for a step displacement pulse that takes 0.2 sec, the abscissa

for a frequency of 1 cps would be changed to 5 cps , and that for 3 cps in the

figure would be changed to 15 cps, etc. The general nature of the similarity

between Figs. 5 and 6 is important.

One can also draw a response spectrum for total displacement, as shown

in Fig. 7. This is drawn for the same conditions as Fig. 5, and is obtained

from Fig. 5 by multiplying each curve's ordinates by the value of ductility

factor y shown on that curve. It can be seen that the maximum total displacement is virtually the same for all ductility factors, actually perhaps

decreasing even slightly for the larger ductility factors in the low frequency region, for frequencies below about 2 cps. Moreover, it appears from

Fig. 5 that the maximum acceleration is very nearly the same for frequencies

greater than about 20 to 30 cps for all ductility factors. In between, there

is a transition. These remarks are applicable to the spectra for other

earthquakes also. One can generalize about them in the following way for

general nonlinear relations between resistance and displacement, for single

degree of freedom structures.

For low frequencies, corresponding to something of the order of about

0.'3 cps as an upper limit, displacements are preserved. As a matter of

fact, the inelastic systems have perhaps even a smaller displacement than

elastic systems. For frequencies between about 0.3 to about 2 cps, the

displacements are very nearly the same for all ductility factors. For

frequencies between about 2 up to about 6 cps, the best relationship appears

to be to equate the energy in the various curves, or to say that energy is

preserved, with a corresponding relationship between deflections and accelerations or forces. There is a transition region between 6 and 20 to 30 cps,

depending on the damping ratio. Above 20 to 30 cps, the force or acceleration

is nearly the same for all ductility ratios.

Structure-Foundation Interaction

Earthquake motions are transmitted through the ground to the foundation

of a structure and then to the structure itself. The interaction between the

foundation components of the structure and the earth upon which it rests are

of particular importance in defining the nature of the forces and motions

transmitted to the structure. Energy absorption can take place at the interfaces between the structure and the foundation, and between the foundation

and the supporting medium. Under certain conditions amplifications of motion

may even occur. The interaction between the foundation medium and the

foundation structure can be particularly complicated when the building is

set into the soil or rock rather than resting upon it.

Design specifications either of the cookbook type, the intermediate

type, or the type solely concerned with environmental and performance

criteria fall short of their requirements if they do not consider the interaction between the structure and its supports, and especially the type of

supports, whether it is pile or caisson foundations, isolated footings, or a mat, or some combination of these.

Nonstructural Components

In buildings, particularly, it is necessary to make a distinction

between those components which are essential parts of the structure in its

resistance to loads and deformations, and nonstructural components which are those parts needed to perform the proper function of the structure but which

221
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\j

— 1

C\J 1? II II o / o / II / a. /\,- /

o- -

II /

a. o

') .-; / V

7i7^

— / uo

u.

>« i=z /

lA// f-

\^^^\

A4po|8A- opnasd ~ A

tosI yt

(—UJo

liJ Q.

COQo

1— " CO

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equ ASo

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a> AS

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T3C < UJ

1- < OUJ Q 0.<n Ml

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CEM

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-J UJo g _l

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222
are not added primarily for resistance to lateral forces. Partitions in a building may be structural or nonstructural depending upon whether they are designed to act as part of the load-carrying framing. However, whatever the

designer's intent may be, all the elements of the structure, whether functional or otherwise, have an effect on the behavior of the building under

dynamic excitation, and must be considered in terms of dynamic response,

strength, and the damage which may be caused by exceeding allowable stress

or deformation limits. Even nonfunctional ornamentation on a building that

can be dislodged because of lateral motion can cause hazard to life as well

as property.

IV. Design Procedures

General Approach

The designer has considerable freedom of choice, in general, as to the

type of resisting structure he will use in the design. He may choose a flexible, energy-absorbing structure which can comply with the ground motion

readily, or he can use a rigid structure to limit the relative deformation

within the structure itself. In one case the strains in the structure are determined primarily by the maximum transient ground displacement or velocity,

and in the other they are determined primarily by the maximum transient ground

accel erati on

.

If the structure is in an intermediate range of stiffness, then its

energy absorbing capacity is of the greatest importance, which involves both

its strength and ductility in some balanced manner. Under these conditions,

one may reduce the strength and increase the ductility, or increase the

strength and reduce the ductility, in both cases arriving at a satisfactory

design. Although the designer must be careful in the determination of these

balances, and must look into the strength and ductility of elements or com- ponents as well as those of the completed assemblage, he can make up for

deficiency in one by an overdesign in the other, in many instances. Constraining the designer to use always highly ductile elements may be unrea- sonably restrictive since it appears possible to design a structure with as much margin to resist failure by making it less ductile but stronger, in an appropriate fashion. It appears unwise to establish design criteria solely

on preconceived notions as to either strength or ductility without considering

the combination of both of these that is required for adequate performance.

Theoretically and to a considerable extent practically, it is possible

to use any material in almost any fashion one chooses to use it, by providing

the proper combination of strength and ductility associated with the particular structural configuration and dimensions, thereby to insure that the

completed structure will be able to perform adequately under the appropriate

loading or motion environmental conditions.

It has already been noted that in many structures it is desirable, and

in fact quite proper and reasonable, for the structure to go well into the

inelastic range of behavior, especially for the maximum or extreme environmental seismic conditions. Different types of framing and different materials

pose a variety of problems for an adequate specification of performance

involving deformations and stresses beyond yield. This has been taken into

account in existing codes in various ways, usually by specifying the relative

intensity of loading to be considered for different types of framing. Each

material must be studied from the point of view of its particular characteristics of strength and ductility, when fabricated into structural members or elements, or when connected together to form a structure. The performance

criteria must be prepared in such a way as to avoid unusual handicaps to any

one type of framing or material, or to give unusual advantages to any other

type.

223
Selection of Parameters

In the light of the preceding discussion, we can now develop a basis

for design of structures, elements or components, where these are subjected

directly to the ground or base motion for which we have maximum values of

displacement, velocity and acceleration. We first proceed with selection of

values of damping. Table 1 is reproduced with some changes from References

[5] and [6], and gives the percentage of critical damping for various types

and conditions of structures or elements, as a function of stress level. It represents the best information available at the present time, but certainly

involves a great deal of judgment and interpretation.

The damping in structural elements and components and in supports and

foundations of the equipment is a function of the intensity of motion and of

the stress levels introduced within the structural component or structure, as well as being highly dependent on the makeup of the structure and the energy

absorption mechanisms within it. For example, a structure with riveted or bolted joints that can undergo relative motion during deformation will absorb

a great deal of energy in friction in these joints. A reinforced concrete

beam that is cracked, where the elements on the two sides of the crack can move relative to one another with the absorption of energy at the faying

surface, will also absorb considerable energy. On the other hand, a homogeneous solid structure or a welded steel structure has relatively small

amounts of lost energy because of play in the joints, and a concrete beam

before cracking has a relatively small amount of energy losses except those

within the material itself. Hence, the degree of damping depends on the

framing and makeup of the structure or elements, and on the material used

and the stress level within the material for the degree of excitation which

it experiences in the shaking motion. For low stress levels and for homogeneous structures, steel or reinforced concrete below cracking levels, the

damping may be no greater than in the range of one-half to one percent. For

stresses at the level of working stresses or at about half the level of yield

point values, the damping may range from about 2 percent for welded steel

structures, for well reinforced concrete structures with only small amounts

of cracking or for prestressed concrete structures, to 3 percent to 5 percent

for ordinary reinforced concrete structures with considerable cracking, and

possibly above 5 percent for riveted or bolted connections, or for wood

structures with nailed joints and the like. At or near yield point values of

stress, the damping may be in the range of about 5 percent for steel structures and prestressed concrete structures that have not completely lost their

prestress, ranging to 7 to 10 percent for ordinary reinforced concrete, and as high as 10 to 15 percent for structures with play in the joints, or for

masonry structures.

The fundamental frequency of vibration, or its reciprocal, the fundamental period, is best estimated by a simple calculation by use of standard

methods of analysis such as are described in Reference [3]. For buildings

simple rules, also given in [3], are often used to approximate the fundamental

frequency, but are generally not reliable for unusual types of framing or for

extremely heavy or extremely light construction.

The ductility factors for various types of construction are more difficult to characterize. They depend on the use of the building, the hazard

involved in its failure, the material used, and the framing or layout of the

structure, and above all on the method of construction and the details of

fabrication of joints and connections. A discussion of these topics is given

in Reference [3] also.

Design Spectrum - Elastic

In either analysis or design for earthquake resistance it is convenient

to use the concept of the response spectrum. A response spectrum developed

to give design coefficients is called a "Design Spectrum".

224
Table 1

RECOMMENDED DAMPING VALUES

Stress Level

Type and Condition of

Structure

Percentage of

Critical Damping

Worki ng stress , no more than about 1/2

yield point

a. Vitalpiping O.Btol.O

b. Welded steel, prestressed

concrete, well reinforced

concrete (only si i ght

cracking) 2

c. Reinforced concrete with

considerable cracking 3 to 5

d. Bolted and/or riveted steel,

wood structures with nailed

or bolted joints 5 to 7

At or just below yield

poi nt

a. Vital piping

b. Welded steel, prestressed

concrete (without complete

loss in p res tress)

c. Prestressed concrete with

no prestress left

d. Reinforced concrete

e. Bolted and/or riveted steel,

wood structures, with bolted

joints

f. Wood structures with nailed

joints

7 to 10

10 to 15

15 to 20

225
In general, for any given area or site, estimates might be made of the

maximum ground acceleration, maximum ground velocity, and maximum ground

displacement. The lines representing these values can be drawn on the

tripartite logarithmic chart'Of which Fig. 8 is an example. The lines showing the ground motion maxima in Fig. 8 are drawn for a maximum ground acceleration of l.Og, velocity of 48 in/sec., and displacement of 36 in. These

data represent motions more intense than those generally considered for any

postulated design earthquake hazard. They are, however, approximately in

correct proportion for a number of areas of the world, where earthquakes

occur either on firm ground, soft rock, or competent sediments of various

kinds. For relatively soft sediments, the velocities and displacements

might require increases above the values corresponding to the given acceleration as scaled from Fig. 8. However, it is not likely that maximum ground

velocities in excess of 4 to 5 ft per second are obtainable under any

ci rcums tances

.

Amplification factors for the various ranges in the response spectrum

were considered in References [5] and [6]. The values determined therein

for a number of earthquakes, with some smoothing and reduction of peaks to

present a reasonably consistent probability of failure (of the order of about

10 percent or less), are given in Table 2. The amplification factors given

in that table are used in connection with Fig. 8, as explained below.

For each of the amounts of damping shown in Fig. 8 or tabulated in

Table 2, the amplified displacements are shown on the left, the amplified

velocities at the top, and the amplified accelerations in that part of the

right-hand side of the figure for which the lines are parallel to the maximum

ground acceleration line, but lie above it. We shall identify these portions

of the line as the amplified displacement region, the amplified velocity

region, and the amplified acceleration region, respectively.

At a frequency of about 6 cps , the amplified acceleration region line

intersects a line sloping down toward the maximum ground acceleration value,

and intersecting that line at various frequencies, depending on the damping.

The intersection is at a frequency of about 30 cps for 2% damping, and the

other lines are parallel to the line for 2% damping. These lines are designated as the acceleration transition region of the spectra. Finally,

beyond the intersection with the maximum ground acceleration line, the

response spectrum continues with the maximum ground acceleration value for

hi gher f requenci es

.

The spectra so determined can be used as design spectra for elastic

responses. The spectra are compl etely- described when the maximum ground

motion values are given for the three components of ground motion, and the

damping is known. When only the maximum ground acceleration is given, the

values used for maximum ground velocity and displacement are taken as proportional to those in the figure, or as scaled by the same scale factor

relative to the maximum ground acceleration compared with 1 g. The amplification factors given in Table 2 and shown in Fig. 8 are

still under study, but it is not expected that majo-r revisions in them will

be required.

Design Spectrum - Inelastic

To use the design spectrum to approximate inelastic behavior, the

following suggestions are made. In the amplified displacement region of the

spectra, the left-hand side, and in the amplified velocity region, at the

top, the spectrum remains unchanged for total displacement, and is divided

by the ductility factor to obtain yield displacement or acceleration. The

upper right-hand portion sloping down at 45°, or the amplified acceleration

region of the spectrum, is relocated for an el asto-pl astic resistance curve,

or for any other resistance curve for actual structural materials, by choosing

226
227
Table 2

RELATIVE VALUES OF SPECTRUM AMPLIFICATION FACTORS

Amplification Factor For

Percent of Critical

Damping Displacement Velocity Acceleration

0 2.5 4.0 6.4

0.5 2.2 3.6 5.8

1 2.0 3.2 5.2

2 1.8 2.8 4.3

5 1.4 1.9 2.6

7 1.2 1.5 1.9

10 1.1 1.3 1.5

20 T.O 1.1 1.2

228
it at a level which corresponds to the same energy absorption for the elastoplastic curve as for an elastic curve shown for the same period of vibration.

The extreme right-hand portion of the spectrum, where the response is governed

by the maximum ground acceleration, remains at the same acceleration level as for the elastic case, and therefore at a corresponding increased total displacement level. The frequencies at the corners are kept at the same values

as in the elastic spectrum. The acceleration transition region of the

response spectrum is now drawn also as a straight line transition from the

newly located amplified acceleration line and the ground acceleration line,

using the same frequency points of intersection as in the elastic response

spectrum

.

In all cases the "inelastic maximum acceleration" spectrum and the

"inelastic maximum displacement" spectrum differ by the factor y at the same frequencies. The design spectrum so obtained is shown in Fig. 9.

An earlier procedure for the definition of inelastic response spectra

for design was presented in Reference [2]. In that presentation, the displacement bound, the velocity bound, and the acceleration bound were determined, respectively, by keeping the displacement constant, the energy

constant, and the force in the spring constant, and drawing the corresponding

maximum response displacement limits.

The revised procedure presented in this report is shown in Fig. 9 for

2% damping, for an el asto-pl as ti c system with a ductility factor of 5. Both

the maximum displacement and maximum acceleration bounds are shown, for com- parison with the elastic response spectrum.

The solid line DVAA shows the elastic response spectrum. The heavy

circles at the intersections of the various branches show the frequencies

which remain constant in the construction of the inelastic design spectrum.

The dashed line D'V'A'Aq shows the inelastic acceleration, and the lines

DVA"A"q shows the inelastic displacement. These two differ by a constant

factor y = 5 for the construction s.hown, but A and A' differ by the factor

VZy - 1 = 3, since this is the factor that corresponds to constant energy, as indicated in Reference [2].

Of course, the el asto-pl asti c or other inelastic response spectra can be

used only as an approximation for mul ti -degree-of -freedom systems.

In the development of a design spectrum one may choose to use an "effective" value of maximum ground acceleration rather than an actual value,

particularly in cases where the higher spikes of acceleration are associated

with very short durations and correspond to velocity changes much smaller

than the maximum ground velocity, or where the duration of the earthquake

motion is extremely short and the influence on failure or inelastic behavior

is thereby lessened.

Vertical and Horizontal Excitation

Since the ground moves in all three directions in an earthquake, and even tilts and rotates, consideration of the combined effects of all these

motions must be included in the design of important structures. When the

responses in the various directions may be considered to be uncoupled, then

consideration can be given separately to the various components of base

motion, and individual response spectra can be determined for each component

or direction of transient base displacement. Calculations have been made for

the elastic response spectra in all directions for a number of earthquakes.

The complete results for the three components of motion for these are not yet

available, but the trends are summarized below.

229
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230
There are several interesting features of the response spectra. For

example, the frequencies at which spikes and valleys occur are generally not

the same for the different directions of any earthquake nor for the same directions at the same site for different earthquakes. The responses for the

two horizontal directions show cross-overs and significant differences in some ranges of frequency. The vertical response is often equal to or greater than

the maximum horizontal response in the high frequency region, but is somewhat

to a great deal less in the intermediate and low frequency regions.

It is suggested that until further information becomes available the

following design criteria be used:

(a) The design spectrum for vertical response be considered equal to

that for horizontal response for frequencies in the amplii'ied

acceleration range or higher frequencies. In other words, the

acceleration bounds are the same for both horizontal and vertical

re s p 0 n s e .

(b) The design spectrum for vertical response be considered equal to

two-thirds that for horizontal response for frequencies in the

amplified velocity or displacement ranges.

Combined Effects of Earthquake Motions

Since the responses for motions in the various directions (horizontal

and vertical) may not occur at the same time, it is considered reasonable to

combine the effects of the several components of motion in a probabilistic

manner, by taking the maximum stress, deflection, or other specific response

as the square root of the sums of the squares of the corresponding responses

to the individual components of motion.

The effects of transient tipping, tilting, and rotation of the ground

during an earthquake have not been studied extensively. An elementary treatment of some aspects of these movements has been given in Section 7.7 of

Reference [3], and the effects of rotation of the ground about a vertical

axis on the accidental torsion in symmetrical buildings, for example, is

given in Section 15.6 of the same reference.

When the responses of the structure or component are coupled, the

analysis becomes much more complex and a three-dimensional (or at least twodimensional) response analysis must be considered. However, data regarding

the simultaneous input motions must be used in such an analysis, and little

guidance is available on this topic.

The motions due to an earthquake occur in both horizontal and vertical

directions in a complex manner. It is necessary to consider the interactions

between the responses in the various directions, and especially important to

consider the interaction between the vertical and the maximum horizontal

response. Vertical loads, and eccentricities of the vertical loads caused

by horizontal displacements, must often be taken into account with especially

heavy structures that carry large masses at or near the points which may

deflect a great deal. Some of the resisting capacity for horizontal motions

may be used up by the secondary effects of the eccentricities of the gravity

loads

.

Quite often, the vertical motions may produce vertical stresses in the

structure or element that exceed by a large amount those stresses due to the

inertial forces corresponding to the vertical acceleration multiplied by the mass of the element. This is true when the frequencies of vibration in the

vertical direction of the element or component are in the range where major

amplification of response can occur.

231
Special Considerations and Quality Control

A number of points are often overlooked in the design of structures or components to resist dynamic motions. A summary of some of the more important

factors, but by no means a complete listing of all of them, is contained herei n

.

One of the factors that is commonly overlooked is the matter of relative

motions between the parts or elements of a system having supports at different points, because the support motions may not occur simultaneously. Hence,

there may be transient relative motions which produce strains in the structure, in addition to the strains produced by the dynamic effects of the overall motion. This is especially important in piping, electric wiring, or other

elements connecting parts of a facility.

Finally, there are a group of items which do not lend themselves readily

to analytical consideration. These concern the details and material properties of the element or component, and the inspection and control of quality

in the construction procedure. The details of connections of the structure

to its support or foundations, as well as the various elements or items

within the structure or component, are of major importance. Failures often occur at the connections and joints because of inadequacy of these to carry

the forces to which they are subjected under dynamic conditions. Inadequacy

in properties of the materials can often be encountered, leading to brittle

fracture where sufficient energy cannot be absorbed even though such energy

absorption may have been counted on in the design.

In order to insure that the intent of the designer is achieved, control

of construction procedures and appropriate inspection practices are necessary.

It is important that the practical aspects of seismic design be emphasized

and that both designers and constructors be fully aware of their importance.

V. Desirable Features of Design Codes

Relation Between Analysis and Design

When the configuration of a structure is fixed by architectural or other

requirements, the designer has a restricted choice in the development of the

strength and ductility required to insure adequate seismic behavior. It is

not always possible to say that some design layouts are better than others for

dynamic resistance, although it is fairly clear that different choices of

framing can lead to vastly different requirements of strength and ductility.

For example, a framed structure is generally less stiff and usually lower in

frequency than a shear wall structure with nearly solid walls providing

lateral resistance. Hence the design forces may be smaller for the framed

structure than for the shear wall structure, although the required ductility

may be larger. Methods of design for the dynamic loadings arising from

earthquakes are in general simpler and better understood for structures for

which there is a great deal of experience. However, unless methods are developed and specifications are devised to take account of new structural

types or of new imaginative architectural designs, such designs will be placed

at a disadvantage relative to more standard designs because of the necessity

for providing greater margins of safety for those designs for which experience

is unavailable.

The methods of analysis, and also the details of the design specifications, have implications on the cost and the performance capability of the

design. If the specifications are unduly conservative the design will not

only be unduly conservative, but may also be forced into a type that is

stronger and less ductile than is desirable. It is difficult to avoid

differences in the degree of conservatism among different types of structures,

and in some cases it is undesirable to do so. Some materials by their nature.

232
including their variability or lack of adequate control of properties, may

require a greater factor of safety than other materials the properties of

which are more accurately determinable and controllable. The margin between

incipient failure and complete collapse may differ for different materials

and may therefore involve a difference in the factor of safety required in

the design.

Basic Function of Design Codes

The designer, as well as anyone else who has a responsibility for the

final structure, has to have some general method of knowing that gross errors

have been avoided, and must have some basis of comparison to insure that the

design is adequate in an overall sense. It is the purpose of building codes

and specifications to perform these functions. However, it is not yet established that building codes can do this kind of job without introducing

constraints and controls that may be a severe handicap on the development of new design concepts and procedures. Where building codes are used to insure,

by ru 1 e-of-th umb methods, that a design is adequate, they embody the result

of experience and judgment and must therefore deal implicitly, if not explicitly, with particular structural types and configurations.

The most desirable type of design code or specification is one which

puts the least restrictions on the initiative, imagination and innovation of

the designer. Such a code might involve only criteria for: (1) the loading

or environment; and (2) the level of response, the stresses and deformation,

or the performance of the structure under the specified loading or environmental conditions. Such an approach need not, and preferably should not,

indicate how the designer is to reach his objective, provided he can show

that he has achieved a structural capability to resist the specified environmental conditions. This approach is generally the one that is now used for

the design of nuclear reactor power stations. Experience over the past

several years in approaching seismic design criteria in this way has indicated

a number of problems, but has also been reasonably successful in avoiding con- straints due solely to the specifications themselves, although there have been

constraints based on the environmental conditions and the stress and deformation 1 evel s all owed .

Seismic Response Criteria

The permissible level of response of a structure must, of course, be

associated with the loading criteria. One cannot be specified independently

of the other. This implies, for example, that different response criteria

are to be associated with the probable earthquake or the historical earthquake from those used for the maximum credible or extreme earthquake. Moreover, for either of these, the response criteria should properly be a function

of the type of structure, the relative cost of repairs for minor damage, and

the hazard in terms of possible loss of life should the structure or any of

its elements fail. Hence, the response criteria could be greatly different

for individual homes than for multistory buildings housing hundreds or even thousands of people, and certainly different even from these for high dams

above large centers of population or for nuclear reactor power stations.

It appears reasonable to establish such criteria in terms of the

consequences of failure, and in relative terms associated with yield points

or buckling loads of similar dynamic limit loads for the particular material

or structural elements used. The aim of the criteria should be consistent

with the basic seismic design philosophy stated earlier. Appropriate performance criteria may well be stated most rationally in terms of probabilities

of failure or collapse associated with various levels of the probability of

the hazard considered.

233
Performance Cri teri a

It is essential that the response levels or maximum stresses and

deformations be limited, for structures, components, and details such as joints and connections, in order to insure adequate strength and ductility

of a structure as well as of its various component parts. However, it is

desirable, in the development of the basis for a performance criterion, that

the designer's approach not be too greatly constrained. For example, it may

be unwise to prescribe limits for both strength and ductility in such a way

that the balance between the two cannot be adjusted to take account of new material properties or new structural types as they are developed. A tradeoff between ductility and strength should be available in the methods that

are permitted, so as to achieve economy without the sacrifice of safety. But

whether one is interested in achieving strength or ductility, or both, the

materials have to be used in an appropriate fashion, and adequate methods of

inspection and control of construction are needed to insure that their use

is proper.

Methods of Analysis

A variety of methods of analysis are now available, ranging from simple

upper-bound static coefficient methods to modal response combinations,

including time history analyses either in the elastic or inelastic range,

and extending to probabilistic methods or methods involving consideration of

random vibrations. It should be possible to use any of these that are sufficiently justified by either general acceptance or by demonstrated

mathematical and physical validity, with requiring that any one particular

method be used. Of course, it is desirable always to allow the use of simple

methods that are adequately conservative. Properly stated criteria, used

with appropriate methods of approximate analysis and design, may make it

unnecessary in many cases to perform detailed and costly analyses, particularly in those instances where the simpler approximate methods insure adequate

margins of safety.

To push these concepts to the limit involves generally the idea of

approximate methods that give reasonably accurate results or, at least,

results that are consistent with those obtained from more precise analyses.

Depending on the degree and extent of the approximation, it may be necessary,

from the point of view of achieving a reasonable degree of precision, to have

special modifications of a general procedure for various unusual structural

classifications. Methods may be used that do not require knowledge of the

period of vibration of the structure, for example, or the methods may involve

an approximate determination of the natural period. Approximate damping

factor determinations may be involved as well.

For the complete development of this type of approach it may be desirable

to explore the possibility of a hierarchy of methods ranging from the crudest

approximation, for very simple structures and structural elements, to more accurate approximate methods, for structures of intermediate complexity, and

to relatively precise and accurate calculations, for extremely complex

structures

.

Special Structures

Although many of the problems associated with the design of special

structures such as nuclear reactors, high dams, schools and hospitals, are similar to those involved in other more ordinary types of structures, there

are some implications of failure of these special structures that require

special consideration in selecting margins of safety and the development of

design procedures and criteria governing them.

Many structures or parts of structures and many items of equipment can 234
be severely damaged without any implication of loss of life or even of major

property damage in an earthquake. For such items and structures it is un- necessary and certainly uneconomical to provide great margins of safety for

unlikely earthquakes. The margin of safety of the provision made for an earthquake of a reasonable degree of probability of occurrence within the

lifetime of a structure need only be great enough to offset the cost of

repairs or reconstruction.

However, structures whose loss of function might cause hazard to life,

or structures which are important to prevent damage to major services, have

to be designed on an entirely different basis. For such structures, an earthquake even of relatively low probability of occurrence, but one that

possibly could occur, should not cause collapse or damage of such a nature

that endangers the health or life of large numbers of the population.

Hence, for such structures, much greater accuracy in procedures and

assumptions is required, and the type of design specification must be more carefully framed and more clearly stated to give an assuredly adequate margin

of safety against failure even for unusual types of structure and framing.

The type of design specification used for major nuclear reactor power

plants has emphasized loading or environmental criteria, and performance

criteria in terms of stress and deformation levels of response. The experience that has been gained with criteria of these types indicates that benefits

are possible for other types of special structures with similar kinds of

design specifications. The advantages are in the encouragement of the designer to explore various types of structures, to consider the use of a variety of materials, and to look for economical ways of achieving the desired

levels of safety and performance.

Detailed prescriptions of methods and procedures were necessary when the

majority of practicing engineers had neither the sophistication nor the com- putational aids to take account of the more accurate methods of analysis and

design that are now available. However, this situation has changed and is

continuing to change at a rapid rate. With the increase in numbers of more highly trained engineers, and with a greater store of knowledge available,-

together with more efficient ways of using that knowledge which have become

possible with the general availability of and accessibility to high speed

digital computers, it appears that it is now possible to make a major change

in our methods of specifying or codifying seismic design.

General Comments and Conclusions

The field of earthquake engineering is relatively new, not much more

than three decades old, and advances in knowledge are progressing at a rapid

rate, not only because of a greater emphasis on analytical and experimental

work in the laboratory, but also because of the availability of more definitive information on earthquake motions, the accumulation of strong-motion

records, and recent accelerated expenditures on research. It is important

that the discoveries from observations, and studies from the research laboratory and the analyst, find their way into practice as soon as possible. But

this is difficult because engineers are traditionally unwilling to take

chances on things that they have not proved out. Too little attention has

been given to methods of demonstrating adequate performance capability for

major structures and structural elements. Although this is not necessarily

a part of our consideration in design specifications, it might well be

desirable to look for ways of determining the capability of completed structures or for ways of proving the performance capability in the design stage

by appropriate methods, so as to encourage the development of more economical

designs and methods.

In order to insure that the intent of the designer is achieved, control

235
of construction procedures and appropriate inspection practices are necessary.

This point cannot be overemphasized. It is difficult to synthesize and distill the collective experience and judgment of the engineering profession

into a set of rules, especially when they cannot be put into a mathematical

formulation. This is a difficulty, however, not only with performance and

environmental criteria but also with more standard types of design specifications of the current and past eras. Nevertheless, it is important that the

practical aspects of seismic design be emphasized and that engineers and

constructors be fully aware of their importance.

It is the intent of this discussion to focus attention on the aims and

objectives of seismic design in such a way as to encourage the development

of methods and practices suitable for structural design of the future; methods

that will permit more freedom and latitude to the architectural and engineering innovator.

VI . References

1. N. M. Newmark, "Design Specifications for Earthquake Resistance",

p. 101-113 in Civil Engineering Frontiers in Environmental Technology , Dept. of Civil Engineering, University of California, Berkeley, 1971.

2. N. M. Newmark, "Current Trends in the Seismic Analysis and Design of

High Rise Structures", Chapter 16, in Earthquake Engineering , PrenticeHall, Inc., Englewood Cliffs, N. J., 1970, pp. 403-424.

3. N. M. Newmark and E. Rosenblueth, Fundamentals of Earthquake Engineering , Prentice-Hall, Inc., Englewood Cliffs, N. J., 1971.

4. N. M. Newmark, "Earthquake Response Analysis of Reactor Structures",

Nuclear Engineering and Design , Vol. 20, No. 2, 1972.

5. N. H. Newmark, "Design Criteria for Nuclear Reactors Subjected to

Earthquake Hazards", Proceedings of IAEA Panel on Aseismic Design and

Testing of Nuclear Facilities, Japan Earthquake Engineering Promotion

Society, Tokyo, 1969, pp. 90-113.

6. N. M. Newmark and W. J. Hall, "Seismic Design Criteria for Nuclear

Reactor Facilities", Proceedings Fourth World Conference on Earthquake

Engineering, Santiago, Chile, 1969, Vol. II, pp. B4-37 to -50.

National Bureau of Standards Building Science Series 46, Building Practices for Disaster Mitigation, Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards, August 28-September 1,

1972, Boulder, Colorado (Issued February 1973).

236
PROCEDURES AND CRITERIA FOR WIND RESISTANT DESIGN

by

Joseph W . Veil ozzi *

and

John J . Heal ey**

Intro duct ion

This paper is intended to review the current status of wind resistant

design, to identify areas where improvements in design methodology and

building code provisions are required and to make recommendations regarding

the acquisition of basic data, research activities, and the development of

analytical and experimental procedures.

Total losses in terms of property damage and loss of life resulting

from severe wind storms will tend to rise in the future as population growth

increases the probability of a storm striking a developed area. In addition,

the need for increased economy in terms of site usage and building materials

adds to the problem by providing more slender structures subject to more severe operating conditions. It is felt, however, that economical structures

with more consistent overall safety and greater resistance to severe wind

conditions can result from a coordinated effort to develop more sophisticated

analysis and design procedures, to accumulate necessary data, and to enact

upgraded building code provisions.

The major sections of this review are as follows:

(1) Severe Storms

(2) Public Safety and Protection

(3) Structural Analysis and Design

(4) Stochastic Methods

(5) Recommendati ons

(6) Ref e ren ces References provide supporting material and background information re- lated to the four major topics in the paper.

Severe Storms

The natural phenomena which produce severe wind conditions of potential

danger of life and property are hurricanes, thunderstorms, tornadoes and

extratropi cal storms. Knowledge of the origin, characteristics and duration

of these wind sources is therefore essential to a program of disaster mitigation. At present, detailed cl i matol ogi cal and structural loading data for

* Associate and Structural Engineer, Ammann & Whitney, Consulting Engineers,

New York.

**Structural Engineer, Ammann & Whitney, Consulting Engineers, New York.

237
severe storms is quite limited, particularly for tornadoes. As the population

grows and built-up areas spread, storm damage and loss of life will naturally

rise due to the increased probability of structures being subjected to severe

loads. The following sections review the data currently available on severe storms, discuss the requirements for basis data, design criteria and code

provisions related to wind-resistant building design, and recommend research

acti vi ti es

.

On the basis of duration, severe storms can be divided into two fairly

distinct groups: relatively long-term storms (hurricanes and extratropi cal

storms) and short term disturbances (tornadoes and thunderstorms).

Hurricanes (tropical cyclones) which affect North America originate in

the middle latitudes of the Western Atlantic Ocean and tend to travel northward along the southeastern and eastern regions of the United States before

they diminish in intensity. The path route of these intense storms (75-150

mph) has an important effect upon the character of the storm. In addition,

factors such as the intake of moisture and cold air, together with path route,

determine if the storm will gain or lose intensity, or perhaps develop into

an extratropi cal cyclone. In general, these latter storms are disturbances

of great intensity covering areas of perhaps hundreds of miles in diameter.

Due to their long duration and somewhat predictable behavior, a good

deal of data has been gathered regarding maximum wind speeds and pressure

distributions of hurricanes and extratropi cal storms. In addition, quite a bit is known about the conditions which spawn hurricanes and the effect of

path upon storm characteristics and duration. In fact, computer models of

hurricanes have been developed for forecasting purposes and for use in con- junction with "seeding" studies to investigate the possibility of reducing a hurricane's intensity and/or altering its path [1]*.

Efforts in the area of strong wind measurements and basic studies of

these long duration storms to provide understanding of their formation and

possible alteration are important and should be encouraged. The most pressing need, however, is in the area of hurricane path prediction to increase

the amount of warning time provided to communities in the path of the storm.

The most devastating winds in this country are those resulting from

tornadoes; yet comparatively little information on tornadoes is available,

especially in terms of measured data. Tornadoes are intense, fairly localized storms composed of a translating vortex with high winds and a center of

very low pressure. A large number of these storms occur each year in the

United States and the amount of damage rivals the total losses caused by

earthquakes [2].

The other type of short-duration storm, the . thunderstorm , is also a source of appreciable short-term violent wind. In contrast to tornadoes,

however, the maximum wind data from this source are included in the wind

records for a particular site, although little is known about gust action

during thunderstorms.

Due to their random occurrence, localized extent and short duration,

data collection for tornadoes is extremely difficult [3]. Most of the

information on tornadoes is related to occurrence and gross characteristics

such as width and length of path and resulting damage. Thom conducted a statistical analysis of records for the period 1953-62 and established estimates of annual tornado probability for one-degree square regions within the

United States, estimates of the probability of a tornado striking a point,

and frequency distributions of tornado path width and length [2]. Tornado

* Numbers in brackets refer to corresponding items in the References.

238
occurrence within a given region varies considerably in number and severity

from season to season and from year to year. Tornado occurrence studies

should be continued and more detailed estimates obtained [4]. Various studies are underway to study the basic mechanisms at work within a tornado and

to describe the characteristics of engineering significance [5]. Research is

being conducted on analytical and experimental tornado models and field

measurements (waterspouts) to investigate tornado formation, changes during

the lifetime and the mechanism for decay. This work may result in increased

warning time to tornado-threatened areas and, possibly, some means of altering tornadoes after their formation.

The objectives of field observations of tornadoes include measurements

of translation velocity, space and time gradients of pressure and velocity,

both horizontal and vertical. Direct measurement of tornado characteristics

with present forecasting systems is virtually impossible because there is

insufficient time to place the required instruments directly in the path of

the storm. As a result, wind speed and pressure differential measurements

can only be obtained through indirect measurements [6].

The significant loads to which a structure is subjected during the

passage of a tornado include the following: direct pressure on the windward

face, outward pressure on the leeward face due to pressure deficit, outward

pressure on the sides, roof and leeward face resulting from the low pressure

in the vortex, and forces due to missiles striking the structure. The initial negative pressure from the vortex and the re-application of positive

pressure when the ambient air pressure returns to atmospheric are actually

applied as significant dynamic loads. The severity of this dynamic effect is

directly related to the translational speed of the storm. In addition, short

duration gusts and pressure drops occur, apparently at random, as the major

loads are being applied. From the standpoint of design, data is required

on the maximum rotational velocity, the translational velocity and the

pressure drop. The significance of the vertical wind velocity in the tornado

core should be investigated as well as the possibility of wind-induced oscillations during the passage of the storm. Bounds should be established on impact loading due to missiles striking the structure.

Existing data on tornado wind velocities and pressure is inconclusive

and displays considerable scatter. Aside from the translational velocity

which can be measured fairly accurately, indirect measurements must be relied

upon for tornado loading data. Some of these indirect measurements are investigation and analysis of damaged structures, observation of ground marks,

scaling from motion pictures taken of the funnel, observation of the shape of

the funnel and missile penetration data. The situation is further complicated

by the height effect within the vortex and the variation of severity within a given storm. In addition, the transition from an indirect measurement to an estimated velocity or pressure usually involves some assumptions which render

the results less reliable. At present, the most reliable indirect measurement

for both velocity and pressure information is the analysis of the motion of

flying debris in the funnel as shown in motion pictures taken during the

storm.

The storm's translational velocity influences two important design

factors as follows: the maximum wind velocity is the vector sum of the

rotational and the translational velocity; in addition, the pressure gradient

is a function of the speed of storm passage. Estimates of the translational

velocity vary considerably from storm to storm and along the path of a given

storm. A range from 5 to 65 mph has been observed with the average velocity

about 45 mph.

A rather wide range of values for the maximum rotational velocity is

found from both measurements and expert estimates. Estimates as high as the

speed of sound (600-700 mph) and indirect measurements of up to 485 mph have

been made. In addition, the speed of gusts has been estimated to be as high

239
as 100 mph. The variation from storm to storm and within a given storm is

considerable. It appears that a reasonable estimate for the maximum speed

in an average tornado would be between 200 and 300 mph. The Atomic Energy

Commission loading requirements for the licensing of nuclear facilities list

a wind velocity of 300 mph [7].

There is even less data on the pressure drops resulting from tornadoes.

Estimates as high as 2000 psf have been made and unofficial measurements as high as 400 psf have been obtained. It follows that little data is available

on the pressure gradients as well. The AEC requirement is a gradient of 3 psi (432 psf) in 3 seconds.

It has generally been considered impractical to design for the severe loading conditions of wind pressure and pressure differentials experienced

during the passage of a tornado. As a result, building code provisions

which directly address the tornado problem are non-existent. In view of the

current efforts to upgrade building construction procedures and practices to

accomplish significant reductions in property losses and human casualties,

this situation should and undoubtedly will be rectified. The development of

special design regulations for earthquake construction should be paralleled

by a similar effort for construction in tornado-prone areas.

An assessment of present building construction indicates that significant

improvements in tornado resistance can be achieved without disproportionate

cost increases by research in the areas of data acquisition, code formulation

and improved building practices. In fact, it appears that reinforced con- crete structures and steel frames designed in accordance with recognized

standards (e.g. ACI and AISC regulations) can already be classified as tornado-resistant construction. While the basic frame is capable of surviving

a storm, the exterior cladding, glass, etc. would suffer severe damage and

improved design criteria are needed in this area. However, more basic modifications will be necessary in masonry and wood-frame construction to insure

that these structures work together as a unit. The performance of reinforced

concrete and rigid frame construction are proof positive of the improved

performance which will result. Due to the large number of residences of

wood-frame construction, improvements in this area will have a great effect

on the total losses incurred due to tornadoes. Areas where new requirements

are needed include the following: anchoring of roof to walls; anchoring of

the structure to the foundation, connection details in general; the addition

of increased lateral stiffness by the use of interior walls as shear walls,

additional bracing, etc. An effective means of alleviating damage due to severe pressure gradients is the addition of venting systems to provide a direct release of the internal pressure during the low pressure phase [8].

The development of venting system criteria requires realistic data on pressure gradients.

One of the factors which argued against tornado-resistant design in the

past was the high values of maximum velocity and pressure drops commonly

quoted for tornadoes in general. Undoubtedly, tornadoes with extraordinary

wind velocities and pressure drops do occur but, like any other natural source of loading, tornado severity is a statistical quantity. Once again

the need is apparent for reliable data so that design criteria can be established in terms of mean recurrence intervals for pressure drops, velocities

and missile impacts for particular regions.

Risk analysis should play a significant role in this area. The anticipated benefits from tornado-resistant design must be weighed against the

added costs. The list of benefits should include, in addition to the

reductions in annual losses in property damage and human lives, other less

dramatic effects such as reduced insurance rates, increased resale value and

the intangible value associated with increased public confidence and security.

Careful study will be necessary to set proper levels of tornado resistance. It is unlikely, for example, that any but the most special structures

240
(e.g. nuclear reactors and public shelters) will be designed to withstand the

direct hit of a major tornado considering the minute probability of such a storm striking a given point.

A coordinated effort is recommended to gather necessary data, develop

design procedures and construction practices and implement the new requirements in practice. It would also seem feasible to institute interim measures

for damage reduction prior to completion of the studies necessary for a more complete understanding and treatment of the problem.

Specific recommendations for this section on severe storms are summarized

be 1 ow

:

(1) The forecasting of hurricane paths must be improved.

(2) Modification effects of seeding hurricanes by both field

experiments and computer studies should be established.

(3) The feasibility of a sensor system for gathering data on actual tornadoes should be investigated.

(4) Existing data on severe storms should be collated and

assembled in a single document relevant to structural

desi gn probl ems

.

(5) An interdisciplinary field team should be formed to assess wind damage for useful data regarding severe storms.

Public Safety and Protection

This discussion is concerned with practical measures to (a) obtain

knowledge through instrumentation studies and field inspection of wind storm

damage, and (b) effect increased protection to life and property by the

recommendation of improved procedures and their implementation through building codes and specifications. Overall, it is recommended that a central

agency, perhaps a national disaster agency, be authorized to provide coordination in the area of wind data acquisition and standardization, post-storm

damage surveys, and formulation of building code revisions. Other important

functions would include monitoring of related research projects, dissemination

of information, educational activities associated with new codes and procedures, and cooperation with other national and international groups.

Instrumentation

A coordinated program of instrumentation for wind effects should be

established to eventually replace the present piece-meal efforts. Steps

should be taken to obtain maximum return from the funds available, i.e.

results should be of benefit to all interested parties--stati sti ci ans , researchers, designers and building code groups. The data gathering process

should be generally upgraded [9]. Consistent standards should be established

for basic wind measurements, e.g. a uniform averaging period should be used

for all official observations. Economic, efficient and, preferably, portable

instrumentation packages should be developed and discrepancies due to instrument differences eliminated. Measurements should be taken in a form suitable

for automated data analysis and the number of available instruments should

be greatly increased. Selected buildings should be instrumented with measuring

devices that are automatically turned on at the onset of a severe storm.

Finally, public cooperation must be encouraged in the area of tornado observations. In particular, programs should be established for the purchase of

valuable photographs and motion pictures of tornadoes.

241
Basic wind measurement studies should include the continuation of climatological programs, and the gathering of information of severe storm characteristics and occurrences. Specific programs to determine wind profiles and

turbulence spectra in cities, especially under storm conditions, should be

initiated without delay.

A good deal of valuable information must also be obtained on windstructure interaction effects and basic structural characteristics such as internal damping levels. It would be valuable to establish data on significant dynamic effects such as vortex shedding, aeolian vibrations, galloping,

and interference effects for various standard sites. Systematic field tests

combined with wind-tunnel investigations should establish valuable design

guidelines applicable to a large number of common situations. Also, the

emerging stochastic design method requires that rather sophisticated

statistical data be available on the wind environment. Close correlation

between wind tunnel results and on-site behavior will be necessary in order

to develop this data.

Assessment of Wind Damage

One of the most valuable sources of data is the inspection of damaged

areas immediately following a severe storm. The existing program for sending

disaster survey teams should be continued and strengthened. Maximum benefits

will result from the ready availability of survey teams composed of experts

from a number of technical and non-technical (e.g. economists, lawyers) disciplines. The cooperation of local officials and the public is essential for

these efforts to be valuable.

Post-storm inspection reports should include a description of the storm

itself (path, intensity), and the type and degree of damage to specific

structures [10, 11]. Where possible, general observations regarding the

response of various classes of buildings and types of construction, should

be made. In some cases detailed analyses may be justified in order to determine wind forces from inspection results or for the study of the dynamic

response of a particular structure. In addition, recommendations on improved

building practices, based on the observed damage, should be made. Conclusions

regarding the actual performance of wind-resistant construction together with

economic analyses of resulting loss reductions, should provide valuable feedback to risk analysis studies and to the financial and insurance aspects of

the wind problem.

Building Codes and Specifications

In order to accomplish reductions in losses from severe storms, significant results and recommendations from field, laboratory and theoretical

investigations must be incorporated into building codes and specifications.

It is urgently recommended that the major codes of practice be updated con- tinually to refer to the most current sources of wind loading information

[12, 13]. In addition, guidelines and background information covering all

aspects of wind resistant design, from risk analysis through occupant comfort

and construction practices, should be compiled for convenient usage.

Basic code modifications, such as inclusion of reliable stochastic

design methods and upgraded cl imatol ogi cal data, will no doubt require time

for implementation. However, there are a number of practical steps which can

be taken immediately to improve code provisions for design against wind

forces. Among these are: (1) stringent requirements for connection details

to insure that the structure and its parts are sufficiently anchored; (2)

provisions for increased lateral stiffness for structures which previously

were primarily designed for vertical loads only; (3) requirements for local

suctions for cladding design to reduce costly and dangerous glass breakage;

242
and (4) provisions for the temporary bracing of new structures throughout

each phase of their erection.

Adequate provisions for control of construction operations and for

modifications of buildings by occupants are required in building codes to ensure that such operations or modifications do not create a wind hazard.

Protection from Mind and Storms

Improvements in the area of storm forecasting and path prediction are urgently required to provide increased warning time to residents of threatened areas. In addition, it may be possible as a result of basic research to

modify or divert storms before they strike heavily populated areas. Other areas where basic meteorological data and predictions should be

fully utilized are that of regional planning and local zoning. Effective

steps must be taken, on both the regional and local levels, to prevent private interests from improperly developing areas whose geographical location,

topographic features or proximity to large bodies of water, make them

particularly susceptible to wind-induced damage.

Finally, special studies should be made in other areas where improvements

in storm protection are possible. For example, the use of ground level portions of buildings for storm shelters should be investigated for cases where

a storm cellar or other shelter is not available. Post-storm surveys often

reveal a small room, closet or bathroom left intact while the remainder of

the structure was destroyed. Also, special attention should be paid to the

wind resistance of communication structures, such as telephone poles, which

play an important role in the maintenance of public services under severe storm condi ti ons

.

The above recommendations may be summarized as follows:

(1) A comprehensive program for development and installation of

instrumentation for measuring the loadings and response of

selected types of structures, and for recording meteorological

data should be accelerated.

(2) Post-storm survey programs should be continued and strengthened,

followed by comprehensive reporting, including a description and

analysis of the physical damage and economic analyses for future

planning and design.

(3) Codes and specifications should be updated to reflect the best

information available on wind loads and effects. Design guides

and criteria to supplement codes should be prepared.

(4) Continuing efforts should be expended to improve the forecasting

of severe storms and their areas of traverse. Better warning

systems are urgently needed.

(5) Further attention should be given to development of control,

operations and management, prior to and during severe storms and

to development of recovery plans and stockpiling of supplies for

recovery operations in the event of natural disasters.

(6) The basis for insurance protection against wind damage should be

broadened and liability considerations should be delineated.

(7) A national data center should be established to provide a central

base for obtaining information on meteorological storm data, storm

damage and economic data, codes and design guides, regional planning

243
and other critical items pertinent to providing protection against

natural disasters.

(8) A comprehensive program of education should be developed for

practicing engineers, architects, code authorities as well as for

students, to present continually updated knowledge for design and

planning against severe storms.

Structural Analysis and Design

The objective of any design approach to wind loading must be the prediction of those modes of structural performance which in one way or another

impair the intended usage or serviceability of the structure. For buildings,

the major factors governing the design against wind are as follows:

Failure due to:

(a) Yielding with excessive permanent deformations.

( b ) Fatigue.

(c) Instability of the structural frame or widespread damage of the

facade or cladding; i.e., the "skin".

Unserviceability due to:

(a) Excessive deflections causing cracking of walls and ceilings,

plus degradation of the structural skin and mechanical systems.

(b) Breakage of gl ass

.

(c) Excessive sway accelerations resulting in discomfort of occupants.

(d) Discomfort to pedestrians in areas surrounding the building.

It is obvious that the return period in years or mean recurrence interval on wind speed that would bring about failure should be extremely large

and much greater than the recurrence interval which renders the structure

unservi ceabl e

.

While the past practice has been to use recurrence intervals of the

order of 50 years for the overall strength design of buildings against wind

1 oadi ng--based more on historical factors in the development of wind loading

than on rational consideration of the loads that the structure should sus- tain--the current practice is to use a design recurrence interval which

accounts for the intended operational usage and life of the structure as well

as the degree of wind sensitivity and risk of human life. In the new ANSI

Standard [12], a 50-year mean recurrence interval is required for the design

of all permanent structures except those that, in the judgement of the

engineer or authority having jurisdiction, present a high degree of wind

sensitivity and an unusually high degree of hazard to life and property in case of failure. In the latter case, a 100-year mean recurrence interval is

required. For structures having no human occupants or where there is negligible risk to human life in case of failure, a 25-year mean recurrence interval is permitted.

Obviously, many structures have no human occupants, but are wind sen- sitive. In such cases, the new Standard is not explicit and the choice of

a design recurrence interval must be based upon judgement. Considerations of

safety (or risk) require that the design wind speed must not be underestimated while it would be uneconomical to grossly overestimate this speed.

244
Wind sensitive buildings are defined in the new Standard as being taller than

200 feet and having a ratio of height to least width greater than 4.

There are no provisions for tornadoes in the new Standard.

It should be noted that the mean recurrence interval on wind speed for

failure of a building or structure is much greater than the design recurrence

interval when conventional factors of safety are employed. In addition,

because complex buildings consist of a considerable number of effectively

independent components, the overall probability of failure is greatly reduced

when the probabilities of failure of the individual members are combined.

Even in simple structures, combination (convolution) of the probability of

substandard structural members will increase the mean recurrence interval for

failure greatly.

The above may appear to be incongruous because, on the one hand, structures are designed with adequate factors of safety above that required to cover variations in material and workmanship, while, on the other hand,

failures are not uncommon. The answers appear to lie, at least partly, not

only in the differences between the design wind loads and the actual wind

loads to which structures are subjected, but also in possible "weak" or brittle points in structures which prevent them from developing their potential

ductility. Examples are inadequate anchors for the roofs and curtain walls

of buildings plus tie-downs for mobile homes. Normally, the strengthening

of potential weak points can be accomplished at a relatively small cost and

is tantamount to a great increase in overall factor of safety.

At present, design recurrence intervals on wind speed are obtained from

one Code or Standard (such as ANSI) while factors of safety and load factors

for structural design are obtained from other codes (such as AISC and ACI).

It must not be overlooked, however, that wind loads and structural strength

are both probabilistic and a rational design can be achieved only by considering the two in combination. In view of the current trend towards taller

and lighter structures utilizing high strength materials and overall lower

safety factors, it would be unconservati ve to rely solely on favorable

unevaluated experience with structures of the past in establishing wind

design criteria for future structures. Even the experience for modern.wind

sensitive structures is very limited since the loading and design criteria

have been in a state of flux over the past 20 years. There is an urgent

need, therefore, for better cooperation among different code writing bodies

in order to establish realistic wind design criteria for combined loading and

to deduce more nearly predictable probabilities of failure.

In the new ANSI Standard for minimum design loads for buildings and

other structures, the procedure for calculating wind loads has been reformulated to facilitate the application of new and more sophisticated wind load

design criteria. The new provisions do not, on the average, result in

different wind loads than have been used in the past, but rather permit the

design of buildings and structures for wind to be performed on a more realistic and rational basis, consistent with the building dynamic characteristics,

location and intended operational usage and life. Major innovations are,

(1) varying load requirements both with the dynamic properties of the building and with the roughness of the terrain, and (2) separate load requirements

for the main frame and for exposed elements such as curtain walls and glass.

The new Standard includes new distributions of extreme fastest mile

wind speeds in the United States based on National Weather Service data. The new data results from the extension of the annual extreme wind speed data

series to the longer records available at airports. Maps are given for 25-,

50- and 100-year recurrence intervals so that a suitable wind speed can be

selected for design as described above.

245
Whereas the old Standard specified the 1/7 power law variation of wind

velocity with height for all terrain, without distinction, the new Standard

presents different wi nd' prof i 1 es and different gust response factors and

ground wind speeds for three different exposure conditions.

In applying the new Standard the calculation of wind loads involves:

(a) the choice of a basic or reference wind speed with a suitably large mean recurrence interval, and (b) the use of three numerical factors, an exposure

factor depending upon the roughness of the terrain, a gust response factor

depending upon both the roughness of the terrain and the dynamic response

characteristics of the building, and pressure coefficients which relate the

external and internal pressures and/or suctions on the various surfaces to

the effective dynamic pressure of the turbulent wind. The pressure coefficients for local design of cladding and curtain walls have been updated to

reflect the current experience regarding numerous structural failures due

to wind forces related to inadequate anchorage of individual panels and roof

elements.

The statistical approach to wind loading seems to be the most promising

and realistic method for formulating design loads for wind resistant design,

and the new ANSI Standard is a step in this direction. Although the annual

extreme fastest mile wind speed is used as a basis for determining loads, the

gust response factors in the Standard are based on stochastic analyses of the

wind speed fluctuations over 60 minute averaging periods, which provide the

necessary constancy and stability for treating wind loading on a statistical

basis. All factors were then referenced to fastest mile averaging periods in

order to maintain consistency with the accepted American practice of using

fastest mile wind speed data in design calculations.

While the new ANSI provisions on wind loading are expected to result in

designs for broad classes of buildings and other structures of more uniform

safety and of greater economy, specialized buildings will normally require

the use of wind tunnel tests and detailed cl i matol ogi ca 1 investigations to assure that the design criteria is both safe and realistic.

For buildings having dynamic properties which tend to make them wind

sensitive, the design procedure should include a dynamic analysis to determine the response of the building to wind gusts. This dynamic analysis

should determine as a minimum the fundamental frequency of the building. A knowledge of the fundamental frequency, together with the wind gust spectra

of the basic wind, permits the design for wind to be carried out on a dynamic

basis using gust response factors [15]. The gust response factor is analogous to the seismic coefficient or "g" factor used in earthquake design

procedures

.

In some areas of the country (e.g. southern California) the code wind

forces exceed the code earthquake forces. The local codes specify static

wind forces to be used in the design of the building and also specify minimum

equivalent static earthquake forces. However, the large investment in a tall

building and the cost of repairing earthquake damage dictates more thorough

dynamic analysis of the response of the structure to earthquake ground motions

of realistic intensity. Because of this design procedure the strength and

form of the structural frame is determined by the earthquake induced deformations rather than by the code prescribed wind forces. It is suggested that

if similar dynamic design procedures were used to determine the response of

buildings to wind forces of realistic intensity, then the design of the

building would be more closely tailored to the actual loading and the net

result would be a design of more uniform safety and thus of greater economy.

The significant problems encountered in wind resistant design are as follows: (1) overturning forces; (2) drift and distortions; (3) curtain

wall, cladding and glass design; (4) anchorage of small structures including

light frame buildings; and (5) serviceability requirements.

246
Overturning forces are not only of considerable concern in regard to

safety, but are very significant in their effect on building drift. Reliable

prediction of these forces is essential to providing building economy and

performance for wind. The prediction and control of drift is critical to

estimating dynamic response and damage control for buildings subjected to severe winds. The effects of interfloor drift are dependent upon the

structural materials and construction techniques used, and therefore research

into the establishment of a design criteria for drift must keep pace with the

development of new materials and/or methods. Static wind design procedures

are not satisfactory for predicting permissible horizontal movements or drifts of buildings. Drift stability calculations, particularly for a tall

structure, should consider all contributory factors including the effects of

column axial deformations, beam and column bending, possible local yielding

or foundation settlement and the secondary movements induced by the interaction of gravity loads and lateral sidesway (P-A effect).

Although failures of such items as window panes may not lead to collapse

of the structural frames, it is important that the design procedure include a rational procedure for design of curtain walls and cladding [14, 16, 19] design. Damage or failure of these items under severe winds can be catastropic

in order of magnitude in terms of dollars and possibly human lives. As part

of the design procedure for buildings against wind, the statistical nature of

structural resistance must be taken into consideration since it is well known

that yield strength of structural steel, fracture strength of window panes,

strength of reinforced concrete, resistance of structural joints, fatigue

resistance of most structural elements, etc. exhibit considerable statistical

dispersion.

In addition to strength and stability requirements, certain serviceability conditions have to be considered in designing for wind loads. These

requirements are intended to ensure the satisfactory performance of the

structure under service conditions. The most significant serviceability

cri teri a rel ate to

:

(1) Lateral deflection or drift of the building, particularly as it

affects the stability and cracking of members.

(2) Relative vertical movements between columns, particularly as these affect the cracking of members and partitions.

(3) Motion of the building as it affects occupancy comfort.

The degree of control of structural cracking under lateral load depends

upon the type of loading. For wind loading the aim should be to keep the

cracking within acceptable limits whereas for severe earthquake loading,

buildings may be expected to develop plastic hinges in the beams and at

col umn faces

.

Stochastic Methods

Stochastic vs. Traditional Design Procedures

Present design practices based on nominal or theoretical values of

resistance and load leave the actual safety completely unspecified. The

designer has no knowledge of the structure's reliability at any stage from

partial failure through total collapse. Consequently, efforts are underway

to establish probability-based design procedures [20] to eventually supplant

present techniques. In stochastic design both loads and resistance are con- sidered realistically, i.e. as random processes. A formulation results in

which quantities are specified in statistical terms, data and testing

requirements are more severe, and more sophisticated procedures are employed.

The result, however, is a rational approach from which meaningful statements

247
can be made at all levels of structural performance [21]. Moreover, the

results can be stated in terms appropriate to risk analysis, i.e. the design

decision process which considers structural performance and economic factors

concurrently

.

Assessment of Structural Integrity in the Wind Environment

Various techniques are available for evaluating the response of a structure to a random wind loading. Input to the problem includes the

dynamic characteristics of the structure and the statistical description of

the wind environment [22, 23]. Required wind data include the wind speed

spectra, i.e. the frequency-wise distribution of wind speed and the structure

of the gustiness specified in terms of the following: the power spectra

distribution of fluctuations in the wind; the correlation between velocities

at different points in the flow; coherence and the probability distribution

of velocity fluctuations. Since data from standard weather bureau sites

would rarely be sufficiently representative of the actual site condition to

justify its use in a sophisticated analysis, meteorological studies of the

site are required. Wind tunnel tests on an accurate topographic model in a boundary layer wind tunnel are the most efficient means of gathering the

required information. Ultimately this wind load data should be generated for

a variety of standard sites including the center of a large city. In this

way, wind tunnel tests would be performed only in cases where the site

peculiarities or the structure itself dictated that a special cl imatol ogi cal

study should be made. The vital role that wind tunnel tests play in this

entire development emphasizes the need for a program to improve and validate

wind tunnel results by comparison studies with full scale tests for all

aspects of wind tunnel testing.

The response of the structure to the site environment can then be evaluated either experimentally or analytically. It should be noted that random

variability in the structure itself must be considered in the analysis. With

respect to wind tunnel testing, it is not feasible to include random building

effects in the test program. Random material properties could conceivably be

treated in an analytical development along with the random load but would add

considerable complexity. As an alternative it may be possible to adjust the

random structure response to reflect inherent structural variability by con- sidering the results obtained by others on the effect of material property

variation on overall structural response.

Boundary layer wind tunnel tests on an accurate aeroelastic building

model can provide the necessary data on structural response to wind and in

the process account for the sequential effect of gust application, the

spatial extent of gusts and the three-dimensional aerodynamic interaction

between the structure and the actual wind environment.

Analytical procedures for evaluating random response are presently

available for a relatively small number of special cases. More sophisticated

statistical dynamics approaches including correlation methods are available

for treating more complex cases. These techniques should be developed to

consider the elastic response of three-dimensional bluff bodies to random

excitation. An independent approach by Monte Carlo procedures can treat the

statistical variation in both loading and material properties in one analysis

by digital simulation.

Full scale tests should be performed to verify the assumptions made in

the wind tunnel and analytical approaches and to develop confidence in the

ability to define the random structural response.

The random response results can be related to design criteria in various

ways. For example, the maximum drift deflection anticipated over a given

248
mean recurrence interval can be established, or, in terms of damage prediction, the expected number of oscillations with maximum deflection greater

than a limiting value, can be found. Similar statements can be made regarding the possibility of severe damage or collapse, floor vibration, glass

breakage, cumulative fatigue damage per year, etc.

Finally, based on the amount of rather complex wind tunnel or analytical

analysis necessary to obtain the results at this stage, it appears that this

treatment would only be justified for a fairly significant structure. As

experience and expertise develops, simpler techniques and appropriate design

guidelines should be established.

Risk Analysis

The next step in this design process is the establishment of the acceptable level of risk for each design criteria, i.e. actually the probability of

unservi ceabi 1 i ty or the probability of failure. A great deal of judgment is

necessary to establish proper levels for the risk criteria since they should

represent a balance between structural safety and overall cost considering

both the economic and social consequences of failure. For example, it would

seem that a general criterion could be established that the mean recurrence

interval corresponding to the collapse load must be longer than the life of

the structure. However, this rule does not necessarily hold in the case of

a structure in the path of a major tornado since economics may dictate a higher risk level for the great majority of structures.

A great deal of effort must be expended before risk criteria can be

established with confidence. Detailed data must be made available, e.g. the

following: statistics on the strength of glass are needed to arrive at

meaningful cladding design criteria; data on human perception and tolerance

for motion are required in order to set limits on tolerable floor vibrations

and building drifts.

Consideration should be given to the use of Bayesian concepts in this

work. This approach has proven to be valuable in other areas by providing

a logical framework for decision making considering cost optimization and

proper utilization of past experience and subjective data.

Establishment of Specifications on Mind Design in Statistical Terms

The incorporation of statistical concepts into design codes is an active

area of which wind loading provisions are only one part. Some of the requirements for this new code formulation are:

The essential features of a probabilistic code should be maintained

but technical difficulties should be minimized.

Safety levels or risk criteria must be established and guidelines

provided so that this legal obligation is not placed upon the

designer.

Safety levels must be high enough to protect the public but not so strict that intolerable construction cost increases result.

The code should cover all common structures and be sufficiently

flexible so that no restrictions are placed on special structures

or more detailed studies.

Codes must be readily amendable for updating as new data is acquired

and new practices evolve.

Regarding wind data, detailed information should be included for a 249

493-610 O - 13 - n
variety of typical site conditions.

It goes without saying that a good deal of work remains to be accom- plished in order to bring this new design formulation into active use.

Summary of Major Recommendations

As a final summary it is recommended that the following tasks be given

priority:

1. A central agency should be authorized to coordinate the wide

range of required activities in an overall disaster mitigation

program

.

2. Basic studies on the origin and behavior of severe storms and

effective measurement programs to obtain reliable data on tornado

loads should be encouraged. The economic feasibility of a sensor

system for gathering data on actual tornadoes should be investigated.

3. The modification effects of seeding hurricanes by a combined

effort of field experiments and computer simulations should be

determined.

4. Design criteria and building practices for tornado resistant

construction should be developed for future implementation.

5. Major codes of practice should be updated continually to refer

to the most current sources of wind loading information.

6. Current knowledge regarding hurricanes and tornadoes should be

compiled into a single document relevant to structural design

against natural disasters. Design guides and criteria to

supplement building codes and specifications should be prepared.

7. The development of stochastic design procedures should be

encouraged, i.e. the necessary analytical and experimental

techniques and probability-based design provisions including

riskcriteria.

8. Statistical data on loading and response characteristics of

buildings should be collected for implementation of the

stochastic design approach.

-9. A comprehensive program of education should be developed for

architects, engineers, builders, students, and code officials,

to present continually updated knowledge for planning and

design against natural disasters.

References

1. Dergarabedi an , P., Chairman, "Summary of Committee Discussion on Tornadoes and Hurricanes," Summary Report , Conference on Wind Loads

on Structures, California Institute of Technology, December 1970.

2. Thorn, H. C. S., "Tornado Probabilities," Monthly Weather Review , Vol . 91 , 1963.

3. Reynolds, 6. W., "Summary of Group Discussion on Tornado Parameters,"

250
Proceedi ngs , Conference on Tornado Phenomenology and Protective Design

Measures, University of Wisconsin, 1970.

4. Flora, S. D., Tornadoes of the United States , Second Edition, University

of Oklahoma Press , 1954

.

5. Abdullah, A. J., "Some Aspects of the Dynamics of Tornadoes," Monthly

Weather Review , April 1955.

6. Segner, E. P., "Estimates of Minimum Wind Forces Causing Structural

Damage," U.S. Weather Bureau Research Paper No. 41, 1960.

7. Doan , P. L., "Tornado Considerations for Nuclear Power Plant Structures,"

Nuclear Safety Vol. 11, No. 4, July-August, 1970.

8. Reynolds, G. M., "Venting and Other Building Practices as Practical

Means of Reducing Damage from Tornado Low Pressures," Bulletin of the

American Meteorological Society , January 1958.

9. Marshall, R. D. and Hsi , G., "Techniques for Measuring Wind Loads on Full-Scale Buildings," Proceedi ngs , USA-Japan Research Seminar on "Wind

Loads on Structures," University of Hawaii, October 1970.

10. Somes, N. F., et al , Lubbock Tornado - A Survey of Building Damage in an Urban Area , U.S. Department of Commerce, National Bureau of Standards,

Washington, D.C. . 1970.

11. Thomson, J. N., et al , The Lubbock Storm of May 1 1 , 1 970 , National

Academy of Sciences for the National Academy of Engineering, Washington,

D.C. , 1 970.

12. American National Standards Institute, "Building Code Requirements for

Minimum Design Loads in Buildings and Other Structures," Latest Edition.

13. Davenport, A. G. and Dalgliesh, W. A., "Wind Loads," Canadian Structural

Design Manual , Supplement No. 4 to the National Building Code of Canada,

Nati onal Research Council of Canada, Ottawa, 1971.

14. Robertson, L. E. and Chen, P. W., "Effects of Environmental Loads on Tall Buildings," Proceedings , USA-Japan Research Seminar on "Wind Loads

on Structures," University of Hawaii, October 1970.

15. Vellozzi, Joseph and Cohen, Edward, "Gust Response Factors," Journal of

the Structural Division . ASCE, Volume 94, ST6, June 1968.

16. Dalgliesh, W. A., "Statistical Treatment of Peak Gusts on Cladding,"

Journal of the Structural Division , ASCE, Vol. 97, No. ST9 , September

1971 .

17. Hart, Gary C, "Response of Building Skins to Wind Gusts," Proceedi ngs , USA-Japan Research Seminar on "Wind Loads on Structures," University of

Hawaii , October 1970.

18. Khan, F. R., "A Rational Method for the Design of Curtain Walls,"

Proceedi ngs , Symposium on "Wind Effects on High-Rise Buildings," Northwestern University, Evanston, Illinois, March 1970.

19. Robertson, L. E., "Engineering Approach to Designing Glass for Wind,"

Architectural Record , February 1967.

20. Ang, A. H. S., Chmn . , "Structural Safety - A Literature Review," Task

Committee on Structural Safety, Journal of the Structural Division , ASCE, Vol. 98, No. ST4, April 1972.

251
21. Davenport, A. G., "Structural Safety and Reliability under Wind Action,"

Structural Safety and Reliability , Pergamon Press, 1972.

22. Bolotin, V. V., Statistical Methods in Structural Mechanics , Holden-Day

Publishing Company, San Francisco, 1969.

23. Hurty, W. C. and Rubenstein, M. F., Dynamics of Structures , PrenticeHall, Inc., Englewood Cliffs, New Jersey, 1964.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards , August 2 8-September 1,

1972, Boulder, Colorado (Issued February 1973).
CRITERIA FOR BUILDING SERVICES AND FURNISHINGS

by

J. Marx Ayres

and

Tseng-Yao Sun

A. Introduction

A serious need exists to develop improved building practices to minimize

disaster induced failures to building services, such as elevators, lighting,

air conditioning, plumbing and fire protection systems, and furnishings, such

as suspended ceilings, partitions and storage racks. This is especially true

in buildings that must be safe and useable after a major emergency, in order

to preserve the order and safety of the general public. This discussion will

be limited to earthquake resistive design, because seismic induced forces on these nonstructural elements very likely exceed those caused by hurricanes,

tornadoes, extreme winds and blast or explosion effects.

In the past, earthquake resistant design has been concerned primarily

with the structural integrity of buildings, and very little attention has

been given to the performance of the nonstructural elements. This has been

based on a design philosophy that the structural frame will be deflected by

the seismic forces and some inexpensive "cosmetic" damage to nonstructural

elements will occur. Until recently [Reference 1], however, the extent of

this damage and its effect on the safety of the building occupants was never

fully evaluated. One of the most significant lessons learned from the moderate San Fernando earthquake, where most buildings suffered no structural

failures, was the magnitude and cost of the nonstructural damage [References

2, 3 & 4 refer to the problem and more detail is presented in Reference 5.

Damage cost statistics on high rise buildings in the Los Angeles metropolitan

area are currently being collected].

The development of corrective measures to reduce this type of damage is

restricted somewhat by our lack of knowledge regarding the behavior of buildings during earthquakes, but most of the problem stems from traditional

divisions of responsibilities between design professionals, and a construction

industry that does not require careful detailing of nonstructural elements.

The structural engineer usually limits his attention to the design of the

structural frame and leaves the detailing of the nonstructural elements to

others on the design team. This generally results in no analysis at all,

because the architect and the other engineers responsible for this design

work, either ignore the problem or lack any real knowledge of its importance

or how to handle it. Many of the installation details of nonstructural elements are deliberately omitted from drawings, because of long standing trade

practices that have left many of these decisions to product manufacturers and

installers. To overcome these problems the structural engineer must lead the

design team to see that all nonstructural elements are fully analyzed and then

detailed or carefully described in the specifications. The entire design team

must then vigorously defend these details from contractor proposed alternates

that do not meet the design intent, and then demand that they be properly

executed in the field.

Principal, Ayres, Cohen and Hayakowa

Vice President, Ayres, Cohen and Hayakowa

253
Figure 1. Traction type elevator. 254
Most building owners, and unfortunately their architects and engineers,

consider building code minimum requirements as adequate protection against

earthquake damage, and they will not increase their capital costs to improve

occupant safety or reduce future repair costs. This firm belief in the

infallibility of building codes is usually badly shaken after each earthquake

But memories are short and the magnitude of repair costs and other post earth

quake difficulties with buildings are not made public, so owners usually

resist added costs for earthquake resistive features that are not spelled out

in a code. Even though we recognize the serious problems in writing codes

and their practical enforcement difficulties, we recommend stronger legislative controls over the work of architects and engineers, methods of plan

checking and approval, and the procedures for construction supervision and

i nspecti on

.

In the following discussion we have limited ourselves to elevators,

mechanical systems, ceilings, light fixtures and storage racks. In each

category, the subject matter has been further subdivided for clarification

and we have used a few drawings to illustrate and support the text. The

suggested corrective measures do not include minimum recommended design

forces. These data are included in various proposed code revisions and in

References 6 and 7.

B. Elevators

Elevators in buildings are primarily the traction type illustrated in

Figure 1, because hydraulic elevators are limited by speed and design to low

buildings. Traction type elevators suffer most of the damage during earthquakes while hydraulic elevators, dumbwaiters and escalators survive with

only minor damage. The damage summary on approximately 2,000 elevators

compiled by the State of California Division of Industrial Safety after the

San Fernando earthquake, see Table 1, provides an excellent indication of the

vulnerability of elevator systems to earthquake forces. Note that 674

counterweights were thrown out of their rails and 109 of these struck cars moving in the opposite direction. Similar damage to elevators has been

reported in other earthquakes [1].

Most cities in our country do not have an elevator code or even elevator

inspectors in their building departments. They depend on manufacturers

adherence to the requirements of the American Safety Code for Elevators,

Dumbwaiters, Escalators, Moving Walks, and to the integrity of the elevators

installers to protect the life and safety of building occupants. Some of

the larger cities do have elevator codes and inspectors, but these codes do

not include requirements for earthquake resistant design.

The following comments relate to components of traction type elevators.

1 . Counterweights and Guide Rails : During the initial shocks of an earthquake, the heavy (7,000 pound

average) counterweights bend or break their roller guides and deform their

guide rails. They become derailed and as the earthquake progresses the free

swinging counterweights inflict additional damage on guide rails, brackets,

spreader beams and cars. In many instances, the loosened counterweights

have hit the roof of cars moving at high speeds in the opposite direction.

A relatively simple corrective measure is to strengthen the guide rails

and install safety shoes on the counterweights similar to that shown on Figure 2. Under normal operation, the safety shoe would ride free of the

rails, but during an earthquake the roller spring would give way to allow the

shoe to contact the rail and carry the lateral forces to the guide rail. The

255
Table 1

ELEVATOR DAMAGE DUE TO SAN FERNANDO EARTHQUAKE

I tem Descri pti on Numbers

Buffers Stinger damaged, guides damaged 2 Cables, compensating Out of grooves or damaged 100

Cables, governor Hang up, cut 20

Cables, hoisting Damaged, jumped deflect sheave (sets) 7 Cables, traveling Hunguporbroken 7 Cars Out of guides, out of alignment 18

Cars Steady rest loose 102

Controllers Moved or damaged 5 Counterweights Out of guide rails 674

Counterweights Out of guide rails, damaged cars 109

Electrical power problems Various causes 4 Electrical Conduit loose 5 Generators moved Some damaged armatures 174

Generators Burned out 5 Guide rails, car Limits not operable, doors not working 7 Guide rai 1 s

,

counterweights Out alignment, bent or broken 49

Gui de rai 1 s , counterweights Brackets broken or damaged 174

Hoistway doors Off tracks, gibs out of sill, draffing 22

Hoistway doors Glass damaged 2 Hoistway walls Bowed and hitting car 2 Hoistway walls Severe cracks, loose plaster and holes 50+

Hydraulic Leaks in casting, plunger rubbing 8

Interlocks and car gate

contact Loose, broken 19

Leveling units Damaged 12

Machine room Floor plates broken, buckled 1 Miscellaneous Annunciators off, brake rods out, lights,

broken selector tapes, fuse blown,

door operator 83

Motor, hoisting Burned out, out of alignment, slipped

rings 13

Pitsflooded Brokenpipes,etc. 7 Pump units Moved, reset leaks 1 Rol 1 er gu i des , counterweights Broken or loose 286

Safeties set 22

Selector Turned over 1 Sheaves, drive Broken or cracked 3 Sheaves, counterweights Broke loose, weight in pit, moved 2 Shoes, guide Broken 9 Victaulic fitting Replaced 1 Dumbwaiters: Cables Off drum 1 Counterweights Out of rails 3 Escalators: Conduit Pulledout 1 Governor chain Off 1 Switches Skirt switches tripped, skirts spread 5 Truss bolts Sheared off 5

256
Figure 2. Proposed coiJnterweight safety shoe.

257
guide rails should be strengthened by using heavier than the typical 8 pound

per foot rolled section. Brackets should be strengthened by the use of

gusset plates or ties similar to that shown in Figure 3, and placed at more frequent intervals

.

Seismic switches to shutdown the elevator during an earthquake and then

lower the cars to the nearest floor should be considered. Manual override

switches are needed for slow speed operation by rescue and maintenance personnel after the earthquake. A more direct solution to the counterweight

problem could be a safety switch mounted on the counterweight guide rail

assembly to shut down the elevator when the counterweight is derailed.

Signal transmission using wireless methods should be considered to avoid

additional wires in the hoistway.

2 . Cars and Guide Rails : Other than damage from derailed counterweights, guide rails for cars sustain very little earthquake damage primarily because they are heavier

than the counterweights guide rails. They are designed to withstand

eccentric live loads in the car during loading and unloading plus braking

forces from the car safeties, and they are fastened directly without brackets

to the structural frame either to the walls of the elevator hoistway or to

heavy spreader beams. It is advisable to strengthen the car guide rails on long spans by installing spacers between the back-to-back rails at midpoints

between the spreader beams, as shown in Figure 4, to increase the rigidity

of both rai 1 s

.

3 . Cables and Hoistways :

The hoist, compensating, and governor cables are tossed around in the

hoistway during an earthquake. They become twisted and snarled, jump out of

their sheaves or guides, and inflict secondary damage when they catch on limit switches and other exposed equipment. Corrective measures that should

be considered include sheave guards to contain cables, and guards on all rail

brackets, ends of landing sills, door-driving mechanisms and other exposed

equipment. Manufacturers should be urged to develop new designs that minimize the use of loose traveling cables in hoistways.

Hoistway construction varies from reinforced concrete used as a structural shear wall, to gypsum board or lath and plaster to form the required

fire rated vertical shaft enclosure. The deflection of the building frame

during an earthquake, with some help from free swinging counterweights, cracks

and breaks up the shaft walls. Debris falling onto the tops of cars inflict

secondary damage, and the movements around the elevator door openings leads

to jammed and misaligned doors. Suggested corrective measures include the

design of more rigid structural frames around hoistways, and door frames that

can accommodate the predicted maximum interstory movements. A defensive solution to falling debris is a screen mounted on the top of cars.

4. Motor Generators : Motor generators must be mounted on vibration isolation mounts to

inhibit noise transmission to the occupied areas of the building. During

an earthquake these isolators fall apart, and the generators are tossed around

the equipment room tearing loose their electrical connections and inflicting

secondary damage on other equipment. A simple and direct corrective measure

is to use molded neoprene isolation mounts, and bolt them to the legs of the

generator and to the floor of the equipment room.

258
1^

Figure 3. Proposed counterweight brackets.

259
^^pze^p^^ ^^^^

-^AK. ^UIP^ JeAIU6

e>eAM^

Figure 4. Proposed spacer befween back-fo-back car guide rails.

260
5. Drive Motors and Traction Machines:

The motors and drives are usually mounted on steel bases and bolted to

the floor, to assure good alignment with the cables, and vibration isolation

mounts are not required because they operate at slow speeds. The reports of

burned out motors, misalignments and slipped rings after earthquakes relate

to the internal design of the unit, and suggests a need for manufacturing

seismic standards.

6 . Selector and Controller Panels : These panels sustained very little damage during earthquakes if they

are bolted to the floor and provided with sway braces at the top. All

electrical components within the panels should be properly secured and

hinged panels provided with positive locking latches.

Most of the hydraulic elevators survive earthquakes with very little

damage because they are essentially hydraulic jacks consisting of a fixed

cylinder and a movable plunger that is attached to the elevator car. Damage

is usually limited to casings out of plumb or with leaks due to ground movements, oil pumps and tanks that shifted causing oil spillage, and minor pipe

leaks. This type of damage is easily avoided by properly securing equipment

to floors and/or walls and using splash proof oil tanks.

Dumbwaiters survive earthquakes with only minor damage, usually derailed

counterweights. Escalators also perform well because they are essentially

two welded steel trusses that span between floors to house the tracks, drives

and motors. Damage is limited primarily to sheared truss bolts at floors and some secondary damage to skirts, switches and governor chains. It should be

noted that this form of vertical transportation, unlike the elevators, even though damaged by the earthquake, remain in place and serve as an emergency

exit for building occupants.

Other more generalized recommendations include:

1. At least one elevator plus the ventilation, communication and

lighting systems on all elevators in a building, connected to

emergency power.

2. Establish preplanned emergency procedures for checking and

restoring damaged equipment.

3. Educate building managers and occupants regarding emergency

procedures for checking and restoring damaged equipment.

4. Educate building managers and occupants regarding emergency

procedures

.

5. After each earthquake require detailed damage reports from

owners and elevator repairmen.

C. Mechanical Systems

Mechanical systems--boi 1 ers and flues, pumps and piping, tanks, fans

and ducts, compressors and chillers, cooling towers, etc. --general ly survive

earthquakes with only minor damage. Heavy tanks will shift or fall over if

their supports fail, and heavy unanchored equipment will move; but most of

the damage to mechanical systems in modern buildings is caused by the use of

vibration isolation devices. Additional research is needed to satisfactorily

261
;l&t2AT;^U \^OU^\lOi<S A'^^eMg'UY

^^^iui^m'T pap

\ H

J ^1

ve^icx^ TO ML^ipe^^

PLAW

Figure 5. Vibration isolation mount with field erected lateral & vertical restraints.

262
-4-

K^'^'ILl&Mr PAP

—fT

V J

L

Figure 6.

PLAki

Vibration isolation mount with factory assembled lateral & vertical restraints.

263
igure 7a. Vibrafion isolation mount with built in lateral & vertical restraining capability.

\^OLKX\ov\ h^ou^rm^:^ ^^acer

igure 7b. Vibration isolation mount with vertical only restraining capability with lateral restraint added

.

264
resolve the fundamental conflict between the requirement to isolate vibrating

equipment and piping from the structure for acoustical control, and the

requirement to tie down equipment and brace piping to minimize damage from

earthquakes

.

Except for the long standing bracing criteria to protect fire sprinkler

piping [8J and some code requirements for anchoring of tanks, very little

thought has been given to earthquake resistant design of mechanical systems.

In the following discussions of proposed corrective measures, all anchors,

bolts, braces, rods, supports, etc., must be designed to resist maximum

anticipated lateral and uplifting loads.

1 . Equipment with Vibration Isolation :

All types of vibration isolation devi ces--from the simplest rubber pads

and molded neoprene devices to the large single and multispring high deflection units, and even air bags--were field tested during the San Fernando

earthquake [5]. Generally, isolators that are not bolted to the equipment

and to the floor are easily damaged during an earthquake. Molded neoprene

isolators sustain less damage than single or multiple spring isolators.

Large diameter spring isolators supporting heavy equipment on upper

floors of tall buildings are easily damaged during an earthquake when the

equipment and its mounting base move with respect to the floor slab. When

the isolators fail, the equipment drops to the floor and connecting piping

and electrical service connections are damaged. Similar secondary damage to

connecting services occurs when suspended equipment with vibration isolation

hangers are subjected to differential movements with respect to the supporting structural member. Vibration isolators lower the natural frequency of

the equipment assembly. Resonant response with large amplitude violent

movements is possible when the predominant period of the structure approaches

that of the equipment assembly.

To provide earthquake resistant design for equipment with vibration

isolation and not interfere with the degree of isolation required for

acoustical control the following corrective measures should be considered.

a. Bolt floor mounted vibration isolation devices to the equipment

base and to the structural slab.

b. Provide lateral and vertical restraining devices around the base

of vibration isolated floor mounted equipment to restrict the

displacement of the equipment, and provide resilient material on the contact surfaces of the straining devices to retard impact

loads. Acceptable devices of this type are shown in Figures 5 and

6, and other devices with varying amount of built-in restaining

capability are shown in Figure 7.

c. Floor mounted equipment isolated by resilient pads or neoprene

isolators and bolted to the equipment and to the structural slab,

do not require lateral and vertical restraining devices.

d. Install vibration isolation hangers for suspended equipment tight

against the supporting structural member.

e. Provide cross bracing between hanger rods on all four sides of

suspended light weight equipment, similar to that shown in Figure

8a.

f. Provide a structural restraining frame around suspended heavy

equipment, similar to that shown in Figure 8b.

265
Figure 8a. Bracing for light weight suspended equipment with vibration isolation hangers.

ALL F^Die^lD^

-L^6k:uur^

^-Hgac: -jir

i -L^^^KW/JT'^'

Figure 8b. Restraining frame for heavy suspended equipment with vibration isolation hangers.

266
2. Equipment Without Vibration Isolation : Equipment without vibration isolation mounts that are well anchored to

supporting structural members sustain little or no damage during earthquakes;

while equipment without anchors will shift and move, often inflicting secondary damage on connected piping and electrical service connections. The

movement of equipment during an earthquake often induces internal damage.

Tanks containing liquids are extremely susceptible to damage during earthquakes. Unanchored tanks tip over, legs and support brackets collapse, they

shift, and sometimes they slip out of supporting straps. These movements

usually cause secondary damage to connected piping.

To avoid this type of damage the following corrective measures should

be considered.

a. Bolt all floor mounted equipment and tanks to the structural slab.

b. Do not use threaded pipe for tank or equipment legs.

c. Strap all horizontal tanks to their saddles, weld lugs to tank at

support points to prevent horizontal movement, and bolt saddles

to structural slab.

d.

e

.

Wherever possible, install ceiling mounted expansion tanks tight

against the supporting structural member.

Any frame supporting elevated tanks or equipment must be provided

with adequate bracing and anchors to the structural slab and walls

Brace all suspended equipment on all four sides, similar to that

shown in Figure 9.

3. Piping Systems : Piping systems generally survive earthquakes with only minor damage.

Excessive pipe movements and differential deflections between piping and

connected equipment or structural elements, often induces failures at

fittings. Screwed fittings are more vulnerable to earthquake damage than

welded or brazed fittings. Pipes are damaged when subjected to excessive

movements at seismic joints, or where a main is free to move and small branch

lines are clamped to a structural element. Fire sprinkler systems are seldom

damaged because they are provided with earthquake bracing.

Earthquake resistant design of piping systems should consider the

fol 1 owing

:

a. Provide sway bracing in both longitudinal and transverse directions

on all pipes 2-1/2 inch and larger following the recommendations of

Reference 8.

b. Do not use branch lines to support or brace large piping.

c. Where pipes are suspended by vibration isolation hangers, provide

sway bracing similar to that shown in Figure 10.

d. Provide flexible joints where pipes pass through building seismic

or expansion joints, or where rigidly supported pipes connect to

equipment with vibration isolation.

e. Do not fasten one rigid piping system to two dissimilar parts of

the building that may move differently during an earthquake; for

example, a wall and a roof.

267
Figure 9. Bracing for suspended equipment.

268
Figure 10. Bracing for piping suspended by vibration isolation hangers.

269
f. Wherever possible, support the weight of the vertical pipe risers

at a point or points above the center of gravity of the riser, and

provide lateral guides at regular intervals.

g. Provide large enough pipe sleeves through walls or floors to allow

for anticipated differential movements.

4 . Air Distribution System : Ductwork, diffusers, grilles, etc., survive earthquakes with only minor

damage. When long runs of large ducts are set in motion by the earthquake,

joints are loosened and flexible connectors at fans are torn. In some instances the ducts have pounded ceiling suspension wires inducing secondary

damage to ceilings. Large diffusers have dropped from ceilings where they

lack proper support.

Earthquake resistant design of air distribution systems should consider

the following:

a. Support horizontal ducts as close as possible to the supporting

structural member.

b. Provide sway bracing similar to that shown in Figure 11.

c. Secure ceiling diffusers and registers to ductwork with sheetmetal

screw. Diffusers connected to the flexible ducts must have positive

ties to the ceiling suspension system.

d. Side wall registers and grilles must have positive ties to the

ductwork and/or to the wall opening.

e. Secure vertical duct risers to shafts or floor openings.

5 . Plumbing, Flues and Chimneys : Plumbing systems survive earthquakes with practically no damage. Water

heaters, that are essentially vertical water storage tanks with weak legs,

fall over, inflicting secondary damage on the pipe connections and flue

vents. Corrective measures for this type of damage are same as that for the

floor mounted tanks. Some water closets seals are loosened during an earthquake and plumbing fixtures are occasionally chipped or cracked.

Heavy masonry stacks and old unreinforced chimneys are classical victims

of earthquakes. The Uniform Building Code and all local codes in the major

cities along the west coast require that chimneys and smokestacks must be

designed to withstand 0.2g. Evidence after the San Fernando earthquake

suggests that the 0.2g requirement may have to be strengthened. The lightweight double wall vents and boiler stacks, where properly installed and

guyed survive the earthquake with very little damage. The only damage occurs

at the vent connectors to boilers, furnaces or domestic water heaters, where

the movement of the appliance damaged the vent connector.

D. Light Fixtures

The various types of light fixtures behave differently under seismic

conditions depending upon their inherent design and their connections to

ceilings. Suspended fixtures that are free to twist and rack are severely

damaged when failures occur in supporting stems or chains and at their ceiling support points. Surface mounted fixtures sustain very little damage if

270
Figure 11 . Bracing for long duct runs.

271
properly installed, and recessed fixtures are damaged when they are not

securely fastened to the suspended ceilings.

The only codes that refer to earthquake resistant design of light fixtures and ceilings are two "rules of application" issued by the City of Los

Angeles Department of Building and Safety, and portions of Title 24 of the

California Administrative Code, that apply to primary and secondary schools.

The Title 24 requirements specify that all light fixtures, ceilings and

supports must be designed to withstand 0.2g lateral force [11, 12]. The City

of Los Angeles requirements describe shake table tests for pendant fixtures

[10], and discuss light fixtures installed in suspended ceilings but do not

specify design lateral forces [9].

1 . Recessed Fi xtures : Recessed fluorescent fixtures are damaged during earthquakes because

they are supported on ceiling systems without positive attachments as shown

in Figure 12. The fixtures pound on the surrounding ceiling elements and

slide or jump off their supports and fall. Most of the damage occurs in

exposed tee bar ceilings where the light weight ceiling elements are easily

moved or deformed.

Corrective measures include properly supported and braced suspended

ceilings as described herein, and the use of commercially available attachment accessories. The fixtures should be supported by and secured directly

to the main runners of the ceiling support system as shown in Figure 13.

They should not be supported by furring or cross runners, or nailing bars in

the case of gypsum board ceilings. Where the locations of the main runners

are not compatible with the lighting pattern, auxiliary support members of

equal strength should be provided.

All recessed fixtures should also be provided with independent secondary

supports attached to the fixture housing and the building structures as re- quired in Reference 12. These supports are a minimum of two 12 guage wires

placed on diagonal corners of each fluorescent fixture, and each wire must

be capable of supporting four times the weight of the fixure. Although this

requirement appears to be redundant after the fixture is secured to a ceiling

support system, it is desirable in the event of ceiling failure or improperly

installed fixture to ceiling attachments.

2 . Surface Mounted Fixtures : Surface mounted fixtures are generally undamaged by earthquakes because

their installation method requires that they be securely tied to the ceiling

system. Some damage has occurred when support clamps of the type shown in

Figure 14a for fluorescent fixtures opened under twisting action, and tee

bar clips of the type shown in Figure 14b loosened. These devices are unacceptable because they depend on friction and good workmanship to hold

the fixture in place. Preferred installations are direct attachments to the

building structure; suspended ceiling installations using positive locking

devices similar to that shown in Figure 15 are acceptable.

3 . Pendant Fi xtures :

All types of pendant fixtures are inherently susceptible to damage

during earthquakes. Their supports fail at ceiling connections, in swivel

joints, at fixture housings, and in supporting stems or chains. The pendant

fixtures also cause damage to suspended ceilings as they rack and twist

during an earthquake.

272
-0

i

Figure 12. Unacceptable recessed light fixtures without positive attachments to ceiling.

273
Figure 13. Acceptable recessed light fixtures with proposed positive attachments to ceiling.

274
Figure 14a. Unacceptable surface mounted fixture clamp.

fliJiiiijlY^

Figure l^b. Unacceptable surface mounted fixture tee bar clip.

275
T-^Ai^, MAIM euuweiz.

~rOF OF l{(^[^r flYTUEB

Figure 15. Acceptable surface mounted fixture tee bar clips with positive locking devices.

276
Contrary to popular opinion, the ball joints in pendant fluorescent

fixtures that were originally developed as an alignment device, do not limit

earthquake damage. It now appears that the conventional supporting assemblies

--pendant hung from free swinging ball sockets--are set into resonance by the

predominant frequencies of the earthquake [5].

After the 1952 Kern County, California earthquake caused pendant fixtures

to fall in several tall buildings in the Los Angeles area, shake table tests

[9] were required on all fixtures installed in the City of Los Angeles. This

pioneering effort successfully established a minimum standard for the manufacture of light fixures, but it also introduced a false security that if the

fixture passed the test it is "earthquake proof". The San Fernando earthquake

provided the necessary field test to indicate the deficiencies in this laboratory simulation. For example, in only five schools approximately 700 "approved"

pendant fixtures were damaged and had to be replaced.

The test procedure is deficient in the following areas:

a. Test is limited to a single one cycle per second frequency;

earthquakes generate a spectrum of frequencies.

b. Test is limited to horizontal movements; earthquakes generate

horizontal and vertical movements.

c. Test is set at 0.2g maximum acceleration; measured accelerations

in the moderate San Fernando earthquake exceeded this limit.

d. Test sample is limited to 8 ft. (two 4 ft. fixtures); common installations exceed 20 ft.

Following the San Fernando earthquake the Los Angeles Unified School

District installed safety cables through every stem of existing and replaced

pendant fixtures. The upper end of the steel cable was roped to the ceiling

support system and the lower end was tied to a large washer inside of the

fixture canopy. The safety cable will prevent the fixture from falling, but

the fixtures will still be damaged in future earthquakes.

We have concluded that there are no safe commercially available pendant

light fixtures and where fixtures must be located below high ceilings they

should be the surface type secured to a supporting grid system that meets

the support and bracing requirements of suspended ceilings. Manufacturers

of lighting fixtures should be urged to develop new pendant fixture assemblies

that can withstand horizontal and vertical earthquake forces and incorporate

the concept of dampened flexibility in stem materials and/or at swivel joints

and connections.

For example, light fixture manufacturers could consider:

a. Stem wall thickness of not less than that of a rigid conduit

(0.109 inches).

b. Ball joints that are restrained from upward movements. One

suggestion is to install a spring in the cup on top of the ball

designed to press against the socket at the bottom of the mounting

plate as shown in Figure 16.

c. Some form of damping should be built into the pendant assembly.

The most effective form of damping, of course, is the viscous type,

but its application in this case would require additional research.

The suggested spring would produce some frictional damping at the

joint, but shake table tests would be required to establish its

ef f ecti veness

.

277
Figure 16. Suggested spring loaded ball joint for pendant fixtures.

278
d. Damping could also provided in the stem by the use of material

similar to flexible conduit but with sufficient tensile strength

to support the fixtures. The exterior of the stem could be coated

with a plastic material to improve its appearance.

Industrial pendant fixtures with long steel rod stems and/or chains are often damaged during earthquakes when they strike each other and adjacent

objects. Failures occur at outlet box connections and when support chains

break or jump out of open support hooks. Corrective measures include the

use of solid link chains and safety latch hooks.

Luminous ceilings are essentially a combination of surface mounted lamps

on one ceiling plane, and a suspended ceiling with plastic diffusers or egg

crate grilles on a lower plane. Lightweight incandescent recessed or surface

mounted light fixtures are seldom damaged during earthquakes. Detachable

fixture accessories such as louvers, diffusers and lenses that are loosened

and fall during earthquakes should be provided with locking catches or screws

and safety chains or cables.

E . Cei 1 i ng Systems

Suspended ceiling systems are damaged during earthquakes because they

are free to swing on their suspension wires and batter against adjacent

partitions or walls. They are also subjected to the damaging pounding forces

from light fixtures, ceiling diffusers, sprinkler heads, etc. Systems that

are flexible at the ceiling plane (exposed tee bar, concealed spline, and

luminous) sustain greater damage than systems with greater rigidity (metal

lath and plaster, gypsum board with glue-on tiles). Wood joist with wood

hangers and nail or glue-on tiles sustain the least damage because the system

is inherently rigid at the ceiling plane and the wood hangers provide good

lateral and vertical bracing.

The only codes that refer to earthquake resistant design of ceiling

construction have been cited herein under the Light Fixture portion of this

paper [9, 11 and 12]. None of the codes call for designs to resist vertical

uplifting loads.

1 . The Exposed Tee Bar, Luminous and Concealed Spline : Exposed tee bar and luminous ceilings are easily damaged during earthquakes, because the system lacks rigidity and supports to inhibit lateral

and vertical movements. The ceiling tiles and recessed light fixtures in

the case of tee bar ceilings, are dropped into the ceiling grids to complete

the ceiling installation. The entire system and light fixtures are set in

motion during an earthquake, and the differential movements at perimeter

walls cause the tees to deform or pull away from their wall supports and

drop out the light fixtures.

Concealed spline installations are inherently more stable than exposed

tee bar and sustain less damage during an earthquake. The tiles are tightly

keyed together by splines so that entire ceiling plane is more rigid and acts

as a unit. Damage usually occurs at perimeter walls where supports are bent

and slit tile are torn enough to fall.

The suspended ceilings also sustain other types of earthquake induced

damage. Pendant light fixtures that are clamped onto exposed tees or tied

to ceiling outlet boxes exert serious twisting and bending forces on the

system. Recessed light fixtures that are not tied to the ceiling, jump or slide off of their supports and batter the surrounding ceiling elements.

Fire sprinkler piping that is supported from and laterally braced to the

279
supporting structure does not move with the ceiling system during an earthquake and sprinkler heads tear ceiling tiles. Similar but less punishing

damage is inflicted by the sheet metal ducts and ceiling diffusers.

Suggested corrective measures should include all of the requirements of

Reference 12 with the following additions or clarifications:

a. Ceiling grid should be braced at regular intervals against lateral

and vertical movements. Some suggested bracing methods are shown

in Figure 17.

b. Do not fasten ceiling system to the surrounding walls or partitions.

Where possible, use soffit to return the ceiling system to the

supporting slab as illustrated in Figure 18a. Where the ceiling

must join a wall or partition, provide an angle wall trim wide

enough to allow for differential movements as shown in Figure 18b.

Main and cross runners should have hangers at the perimeter so that wall trims do not support the ceiling.

c. Cross runners should be fastened to the main runners, using lacking

clips of the type shown in Figure 19, or similar devices designed

to prevent cross tees from pulling or twisting out of the main

runne rs .

d. Ductwork and piping systems in the ceiling space should be braced

against lateral and vertical movements.

e. Avoid the use of pendant light fixtures.

2 . Gypsum Board with Glue-On Tiles, and Lath and Plaster

These two types of ceiling systems are considerably heavier and more

rigid than the tee bar or concealed spline systems and they usually survive

earthquakes without significant damage. Typical failures in the gypsum

board ceilings are dropped or loosened tiles at perimeter walls. Indications

of damage due to vertical uplifting forces were noted during the San Fernando

earthquake. Loosened hanger wires allowed a ceiling to sag because expansion

shields and powder actuated fasteners were pulled from the supporting slab;

and in another installation, nails were pulled out of nailing bars and nail

heads were pulled through the gypsum board.

Plaster ceilings usually are undamaged except at perimeter walls where

they are subjected to differential movements. In large irregular shaped

ceilings where furring channels are wired together and not rigidly connected,

plaster cracking occurs because of flexible corner supports. Where long rows

of recessed light fixtures and linear air diffusers divide large plaster

ceilings the differential movements of the two ceiling sections will allow

the lights and diffusers to lose their supports and fall.

Suggested corrective measures include all of the requirements of

Reference 12 with the following additions or clarifications.

a. Ceiling system should be braced at regular intervals against

vertical movements and provided with lateral braces at the

perimeter.

b. Gypsum board ceiling should be reinforced at nail points by the

use of steel nailing strips.

c. Furring channel joints in irregular shaped ceilings should be made

up using rivits, bolts or welds. Corners should be braced so that

they do not pivot.

280
(^TH^i^ U(r?re^ Ae^vg

Figure 17. Proposed methods for providing lateral & vertical bracing of suspende ceilings.

281
<7

iatM.Ll.lf[ii ]!iiiir'i!'Mil!llilH

Figure 18a. Use of soffit to separate ceiling from wall.

nr/ 1 V'i fi n fTr,^rT7T]}"niiI[II!inTllliira

WALL-

-VVALLT^IM

Figure 18b. Use of angle trim to allow for movement at wal

282
Figure 19. Locking clip to fasten cross runner to main runner.

283
d. Large ceiling areas separated by rows of linear diffusers or light fixtures should be held together with rigid ties and the

diffusers and light fixtures should be secured to the ceiling

sy s tern

.

3 . Wood Joists with Nail or Glue-On Tiles :

Wood joist with nail or glue-on tiles is the most earthquake resistant

of all ceiling types. The ceiling plane, made of wood striping nailed to the

2x4 joists, is very stable and the wood hangers, nailed to the wood joists

and the structural members above, provide vertical and horizontal bracing.

Where wood hangers are substituted for hanger wires, the ceiling should be

braced vertically and horizontally in the same manner suggested for the

plaster ceilings.

F. Storage Racks

It is surprising that more people are not injured during earthquakes

when storage racks dump their contents, totally collapse or fall over. Some

racks collapse because they lack internal strength, and others because they

are not anchored to floors and/or externally braced to a structural member.

In some instances parallel rows of racks with strong ties at their tops,

survive earthquakes with no damage. Free standing unanchored rows of tall

storage racks or cabinets usually fall over like a row of dominoes.

The only code that refers to earthquake resistant design of storage

racks is Title 24 of the California Administrative Code [11]. It requires

that all case work in schools to be anchored to the floor and/or laterally

braced at the top to resist 0,2g applied in any direction. These requirements

must be strengthened and extended to all types of buildings, with special design consideration for important racks carrying emergency power batteries,

communication equipment and vital records.

Earthquake resistive design for storage racks should consider the

fo 1 1 owi ng

:

1. Racks must be designed with lateral bracing and anchor bolts that

can withstand anticipated lateral and uplifting loads,

2. Rows of racks should have rigid ties installed at the top of the

racks to brace and stabilize the entire installation.

3. Racks placed along walls should be anchored to the wall to avoid

battering between the wall and the rack.

4. Develop educational programs that suggest face bars on high shelves.

\ References

1. Ayres , J. Marx, Tseng-Yao Sun and Frederick Brown, Report to Nonstruc - tural Damage to Buildings Due to the March 27, 1964 Alaska Earthquake . Submitted to U.S. Army Engineer District, Anchorage, Alaska, 1967, p. 440 manuscript. Scheduled for publication by the National Academy of

Sciences, Washington, D.C., in 1972 as part of the Engineering Volume,

one of eight volumes titled The Great Alaska Earthquake of 1964 . Total

manuscript published by Consulting Engineers Association of California,

1971, and mechanical section only by Building Systems Design , November

and December 1971.

284
2. The San Fernando, California, Earthquake of February 9, 1971, Geological

Survey Professional Paper 733 , U.S. Government Printing Office,

Washington , D.C. , 1971

.

3 . Engineering Features of the San Fernando Earthquake February 9, 1971 , Paul Jenni ngs , Edi tor . Cal i f orn i a Institute of Tech no 1 ogy , Pas adena , Cal i forni a , 1971.

4. Steinbrugge, Karl V., et a1 . , San Fernando Earthquake February 9, 1971 . Pacific Fire Rating Bureau, San Francisco, California, 1971.

5. Ayres, J. Marx and Tseng-Yao Sun, Nonstructural Damage . Submitted to

Earthquake Engineering Research Institute and National Oceanic and

Atmospheric Administration, 1972, p. 176 manuscript. Scheduled for

publication in late 1972 as part of final report of Reference 2.

6 . Earthquake Resistive Design Construction for Mechanical Systems, Light

Fixtures and Ceilings , J. Marx Ayres and Tseng-Yao Sun, unpublished

report submitted to Los Angeles Unified School District, July 1972.

7. Sharpe, Roland L. and Ronald P. Gallagher, Needed: Seismic Design of

Mechanical and Electrical Equipment , Kaiser Steel, 1972.

8 . National Fire Codes, Volume 6, Sprinklers, Fire Pumps and Water Tanks , Section 3150. National Fire Protection Association, Boston, Massachusetts , 1971 .

9 . City of Los Angeles, Department of Building and Safety, Rule of General

Application, RGA 12-69 . Standard for Lighting Fixture Supports, 1969.

1 0 . City of Los Angeles, Department of Building and Safety, Rule of General

Application, RGA 3-67 . Minimum requirements for ceiling suspension

systems supporting luminuous ceilings and acoustic tile ceilings with

and without lighting fixtures, 1967.

1 1 . Title 24 California Administrative Code, Special ^fgujations Architecture and Construction , Register 69 No. 42. Table T21-107(b), 1969.

12. State of California, Office of Architecture and Construction , Office

staff notes interpreting Reference 10 for application in schools, 1970.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation, Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards , August 2

8 -S eptember 1,

1972, Boulder , Colorado (Issued February 1973).

285
BEHAVIOR OF STRUCTURAL ELEMENTS

A REVIEW

by

B. Bresler*

A. Introduction

This review is intended as a brief summary of the current knowledge

regarding behavior of structural steel and concrete el ements under seismic

and similar extreme conditions, such as hurricanes, tornadoes, etc. In considering the special structural design requirements for buildings under

extreme natural hazards, particular attention is placed on the earthquake

resistant design. This is done for several reasons: (1) earthquakes present

an extreme natural hazard, as exhibited by the Alaskan earthquake, 1964, and

San Fernando earthquake, 1971; (2) structural design requirements for assuring safety against hurricanes and similar effects are also similar to the

requirements for earthquake resistant design; and (3) most of recent research

has been directed toward increased knowledge of structural behavior under

seismic conditions.

Current practices in design and construction and development of improved

practices from documented research are discussed and some suggestions for

further high-priority research activities are included.

B. General

B . 1 Loading Characteristics

Most structural design criteria (including design loads, methods of

analysis, evaluation of stresses, deflections, and load-carrying capacities,

as well as specifications and codes) have been developed assuming that the

applied loads were static (or quasi -stati c ) and that loads which might produce

distress (or failure) would increase monotoni cal ly and remain proportional to

each other throughout. In general, separate loads may change independently,

may be repetitive, and can be described more properly as "variable repeated

loads."

Under normal service load conditions structures behave essentially

elastically. Buildings designed for such conditions, excluding earthquakes,

hurricanes, and similar extreme conditions, are not likely to experience

adverse effects of variable repeated loadings, particularly when the ratio of

live load or normal wind load effect to dead load is one-half or less. It has been generally concluded that under such loading conditions design based

on monotoni cal 1 y increasing loads is satisfactory.

Two traditional design approaches are based on this concept. The working stress method of design, based on service loads and allowable stresses,

is most commonly used in design of steel buildings. The "ultimate strength"

method of design, based on "ultimate" loads (prescribed multiples of service

loads) and "ultimate capacity" (calculated strength) of structural elements

is now widely used in design of concrete structures.

*Professor of Civil Engineering, University of California, Berkeley,

Cal i f orni a

.

286
A more comprehensive and logical design approach, now gaining wide

acceptance in Europe and U.S.A., is the "limit state approach." This permits

recognition of the different critical conditions of loading and of structural

response as independent "limit states" and therefore may be more adaptable to

design for extreme loadings.

A basic characteristic of severe earthquake and extreme wind loadings is

the cyclic repetition of high intensity reversible loads. Under such conditions design for monotoni cal ly increasing loads to failure is not sufficient

to ensure limited damage and safety.

Two principal differences are introduced by earthquake induced loadings:

(a) Time-History of Loading . Because of the dynamic nature of this

loading, it is usually characterized by repeated load reversals and excursions

into the inelastic range. The consequence of such cyclic loading is the

possibility of deterioration in load carrying capacity, (low-cycle fatigue),

even when the number of such repeated overloads is small.

For rigid structures, the period may be as low as 0.1 sec., and the

number of inelastic reversals may be as great as 100. For flexible structures, the period may be as high as 10 sec, and the number of inelastic

reversals would be small, perhaps only 2 or 3 during the earthquake.

Any deterioration due to alternating load reversals may cause changes

in the stiffness, as well as strength, of the structural elements. Because

the overall dynamic behavior depends on the stiffnesses of the various elements in the structure, knowledge of the time-dependent stiffness of the

elements becomes an important consideration in the analysis and design of

the structure.

(b) Energy Absorption and Dissipation . Seismic as well as hurricane

loadings are usually associated with a release of large amounts of energy

causing randomly varying ground motion or movements of large masses of air

at high speeds. The energy so released must be dissipated in various ways--

by ground, by man-made structures, by wave-action in large bodies of water,

etc. The amount of energy dissipated in the structures by inelastic action

is often preferable to the very high intensity of stress (or force) which

would be generated in the elastically behaving structures exposed to large

transient inputs of energy. Therefore, knowledge of inelastic behavior of

the structures from the point of view of energy absorption and dissipation

i s essenti al

.

B . 2 Special Requirements

In the presence of inelastic deformations the load-displacement relationships are described in terms of ducti 1 i ty . Ductility factor y is

generally defined as the ratio of total de f ormati on to elastic deformation at

yield. Fig. B.l.

The fundamental concept of ductility is based on the material behavior

under essentially static loading, and for an el asto-pl asti c material it is a valid measure of energy absorption capacity. This concept of material

ductility (or strain ductility) has been extended with reference to curvature

ductility or extensional ductility --in which case the effects of cross section

shape and size are combined with material characteristics, and even more generally, the concept of ductility has been used with reference to overall

displ acements--i n which case the overall geometry of the structure and the

loads upon it are involved in the evaluation of ductility.

Based on the assumption that the building will undergo significant

inelastic deformations during a severe earthquake and in this manner will be

287
capable of dissipating appreciable amount of energy without fai1ure--the

usual design criteria specify approximate methods for calculating seismic

forces which rely on the ductility of the structure.

Use of ductility factor as an index of satisfactory performance under

cyclic reversed loading is not reliable. First, ductility does not adequately

describe behavior of structural elements under cyclic loading, particularly

when the element exhibits a decrease in stiffness or capacity to resist loads

with increasing number of cycles. This is often the case for concrete structures. Second, ductility does not adequately describe energy dissipation

capacity of the structure; the ideal el asto-pl astic behavior, for which the

concept of ductility was originally established, often overestimates energy

dissipation capacity. Finally, it may be difficult to relate ductility to

damage, particularly cumulative damage under cyclic loading.

In the presence of cyclic loading the load displacement relationships

are described by hysteresis loops. In the simplest case, say an ideal elastoplastic material, the hys teres i s loop for a constant plastic deformation in

each half- cycle becomes a parallelogram, Fig. B.2. While elasto-plastic model

is a good representation of the behavior of steel under monotonic loading, it

is not valid for cyclic reversed loading. Tests have demonstrated that inelastic behavior upon load reversal is strongly non-linear and is particularly

sensitive to the previous inelastic straining. This nonlinear characteristic

of steel is usually referred to as "Bauschinger effect". Fig. 8.3

Two types of hysteresis loops have been observed in tests of structural

elements under fully reversed repeated loads. The first is characterized by

a stable set of hysteresis loops, in which the loop does not change significantly with successive cycles of loading. Fig. B.4. The second, is characterized by "degradation" or changes in stiffness, and energy dissipation with

successive cycles of loading. Fig. B.5. Various mathematical models have

been proposed to represent behavior of structural elements having either the

first or the second mode of hysteretic behavior. Fig. B.6-B.8.

A convenient parameter in evaluating ene rgy dissipation is plasticity

ratio TTjj, defined as the ratio of residual plastic deformation in any given

cycle to the elastic deformation at yield. Fig. B.l,

The area under the hysteresis loop in a given half-cycle (dissipated

energy) is clearly related to tt^ for that half-cycle. In reporting experimental data values of y and ttcI for each half-cycle would provide a valuable

basis for generalizing test results. Fig. B.l. Energy dissipation may also

be useful in defining cumulative damage.

For an ideal alternating cyclic loading the hysteresis loop becomes

stable and values of maximim displacement Aj and corresponding loads Pj

become constant. Fig. B.4. When either the loading varies or when material

is sensitive to prior history the loops for consecutive cycles will vary.

Even with constant values of A there may be a change in the corresponding

resistance, usually a reduction in resistance due to some cumulative damage

with cycling. Fig. B.5.

Such reduction in resistance can be expressed as a ratio Xj which is

the ratio of resistance Pj to the initial value P-j , assuming cyclic variation

with constant maximum deformation A for each cycle.

B . 3 Types of Elements and Loading

The types of elements considered in this review are identified by con- sidering the most common types of structural systems in buildings. These

are: braced space frames, unbraced momen t- res i s ti ng space frames, and box

systems. In an unbraced moment-resisting space frame the lateral force

288
A

FiR. B.2 Ideal Elasto-Plasto Fig. B. 3 Elasto-Plast ic Model Model of Load Reversal With Bauschineer Effect

289
Fig. B.4 - Ideal Cyclic Loading - Stable Hysteresis Loops.

Flg. B.5 Hysteresis Loops for Reinforced Concrete

Column Subjected to Shear leversal.

P - shear, A - lateral displacement

(from Ref. 12)

290
291
Wg. B.7 Cubic Skeleton Curre and Fysteresls Loops

(from Ref. 73)

Fig. B.8 Degrading Stiffness - Hysteresis Loops

(from Ref. 82)

292
resisting system consists of beams, columns, and joints. The joint--which is

the entire assemblage of members and connections at the intersection of the

members--is a particularly important element. In a braced space frame the

lateral force resisting system may consist of frames with X-bracing, frames

with shear walls, or a structural core (box or tube). A "box" system is a structure in which the walls carry both lateral and gravity forces.

In evaluating the type of loading on a given element it is important

to define the loading history. Because peak loadings are particularly

important - it is necessary to define the maxima and minima values which

indicate the range of variation in loading and possible magnitude of reversal.

For example, in a multistory building, the gravity effect may be dominant in

the upper stories, where the lateral force per story is small, and relatively

less important in the lower stories. For these reasons moment and shear

reversals in the beams are unlikely in the upper stories. For a realistic

evaluation of loading - envelopes of axial and shear forces and bending

moments should be constructed indicating the extreme values produced by com- binations of gravity and lateral forces.

For interior columns of moment resisting frames gravity loads produce

essentially axial compression in the columns, as the bending moments due to

unbalanced loads or unequal adjacent spans are relatively small. Lateral

forces are likely to produce full reversals of shear and moment; for this reason loading conditions on columns are often more critical, from one point

of view of cyclic behavior. Loading conditions for interior joints in moment

resisting frames generate reversals in column and beam sections adjacent to

the joint, and cause high shears in the joint region. Some of the detailed

mechanisms in resisting these forces will be discussed in greater detail in

subsequent sections.

In moment resisting frames ductility in lateral displacement is developed

through localized inelastic deformations which occur at certain critical re- gions of the structures, Fig. B.9. Usually these critical regions are located

at the points of maximum internal forces or moments. Typically these critical

regions are located in the beams (or girders) of the frame, or in the connections of the beams to the columns. This is not necessarily the case--but

follows from the recognition of the fact that extensive yielding in the

columns, particularly under combined bending and compression, may quickly

lead to frame instability and collapse. Therefore, the usual design choice

is to prevent formation of plastic hinges in the columns, and to allow them

to form only in the beams or girders.

Localization of critical regions in which large inelastic deformations

can take place suggests that a large portion of the structure may be free of

such inelastic deformation and these "uncritical" portions may be designed in

a more-or-less conventional way. There is some danger in this premise.

Clearly in order to develop the full potential of the critical regions--a

redistribution of forces (moments) may occur resulting in high tension,

compression, or shear forces wehre they do not occur normally. Capacity to

carry the redistribution forces must be provided in order to prevent premature

failure and to develop fully the potential local ductility. In providing the

capacity to carry these redistribution forces the possibility of cyclic load

reversal must be considered.

Typical critical regions which may develop inelastic deformations under

the action of seismic loadings are shown in Fig. B.9. They are:

1. End of beam and connection between beam and column where large

negative moments can develop; likely location for plastic hinge;

partial shear and moment reversal possible.

2. Midspan of beam where maximum positive moment may result in a plastic hinge; shear reversal possible in this region; also

293
Fig. B.9 Critical Areas in Ductile Moment Resisting Frames

(from Ref. 31)

294
partial moment reversal is possible.

3. Ends of columns where maximum moments develop due to lateral

forces; while normally plastic hinges should not be allowed to

develop in these regions, values of maximum column moments closely

approaching plastic moment capacity can be expected; these moments

are likely to undergo full reversal; high shears can be developed

(based on opposite sign moments at the column ends) - and these

shears can undergo full reversals.

4. "Joint" regions (for steel frames may be referred to as panel

zone) - undergo very high local shears; mechanisms of inelastic

behavior in steel and in reinforced concrete are different - but

in both cases may be very important; full reversal of shear is

likely; 1 ocal deformations may be a significant part of rotation

at joint - increasing lateral displacements of frame.

5. Midheight of columns where bending moments are low, but full

reversal of shear is likely; this is more important in reinforced

concrete frame design than in steel design.

B .4 Material Characteristics

Behavior of structural elements under variable dynamic loading conditions

can be predicted if material behavior is properly taken into account.

Under dynamic loading higher values of yield stress have been observed

in structural steel and higher values of compression strength have been

observed in concrete. Typically, yielding of steel under dynamic loading

conditions may occur at stresses 10 to 15% higher than under slow monotonic

loading conditions while values of modulus of elasticity of steel are essen- tially unaffected by the rate of loading. There is some question as to

whether the steel under cyclic loading achieves the higher yield strength in

other than the initial yield cycle.

Under monotonic loading and for large inelastic deformations strainhardening of steel may significantly influence strength and deformation of

the structural element. Under normal (relatively slow rate) cyclic reversed

loading structural steel and reinforcing steel exhibit a "work-softening"

characteristic, known as "Bauschinger effect," Fig. B.3. This greatly reduces

the effective (tangent) modulus of elasticity at stress levels approaching

yield strength after the first reversal. This reduction in modulus may result

in local buckling at substantially lower stresses than yield strength, particularly in cases where full reversal occurs. Fig. B.IO shows results of a typical cyclic test on a specimen of reinforcing steel. After first reversal

the stress-strain becomes nonlinear almost immediately, certainly at a much

lower compression stress level than the initial yielding in tension. In subsequent cycles all loading curves in either tension or compression are highly non-linear, while the unloading diagrams are essentially linear.

Effect of aging can be seen in the figure: portion ABCDE - was obtained at

initial test time; EFG - was obtained 20 days later; GHJ was obtained 73 days

after initial test.

In many structural elements the stresses are complex: in most steel

structural elements the stresses are biaxial, and locally triaxial states of

stress often exist. Similarly in concrete, the stresses are biaxial or triaxial. Effect of state of stress on yielding and failure under repeated

reversed loads is not well established. For this reason, i.e., because of

lack of information, the criteria for yielding and failure developed for

monotonic loading, are used for cyclic reversed loading as well. Thus the

Hencky-Mises yield criterion for steel is usually used.

295
Tension

Stress

Fig. B.IO Typical Stress-Strain Hysteresis

Loops for Reinforcing Steel Under

Complete Reversal Cyclic Loading

(from Ref. 58)

296
It is well recognized that steel may fail in a relatively brittle manner

when subjected to biaxial or triaxial states of stress, when at low temperatures, and particularly when these conditions exist in conjunction with some metallurgical defects, welding defects, or local imperfections. These conditions are more likely to exist when welded steel elements using plates of

large thickness are used in a building. Behavior of such elements under

repeated reversed loads requires further investigation.

Compression strength of concrete under dynamic loading conditions may

be 15 to 20% higher than under normal test conditions and values of modulus

of elasticity of concrete under dynamic conditions are about 5 to 10% higher.

No one criterion of failure under combined stresses has been widely accepted

for concrete. The octahedral shear criterion seems to fit the available data

the best. For the special case of cyclindrical biaxial compression the

following empirical equation

is widely used. In this equation feu is the longitudinal compression strength,

f^ is the radial compression stress, and f^o is the uniaxial compression

strength when fy = 0.

With repeated loading at fairly high stress levels, concrete (and

masonry) develop microcracks which reduce the stiffness of these materials.

Studies by Gerstle et. al., on plain concrete in compression have shown [27]

that after one or two cycles the stiffness of the concrete indicated by the

slope of the reloading stress-strain curve diminishes greatly. On the other

hand, cycling with partial unloading in each cycle does not produce a significant reduction in stiffness.

Figure B.ll shows test results comparing stress-strain diagrams of hardrock and lightweight aggregate concretes, both having a nominal compressive

strength of 5 ksi [39]. Initially, the normal -wei ght concrete has greater

stiffness than the lightweight and it peaks (reaches the compressive strength

level) at a slightly lower strain. For the particular concretes tested, the

lightweight crushed suddenly at the maximum stress level, while the normalweight concrete failed gradually showing a typical descending branch. In a similar test of two concretes - lightweight and normal -wei ght , having com- pressive strength of only 3 ksi, the lightweight concrete failed more gradually than the normal -wei ght one.

The sudden (brittle) failure of the lightweight aggregate concrete is

due to the fact that for this concrete (and aggregate) failure occurs through

the aggregate when its strength is reached. Reduction in ultimate strain

capacity of such lightweight aggregate concretes limit the use of high

strength steel reinforcement and the ductility of reinforced concrete members.

Additional information is needed for evaluating behavior of lightweight

aggregate concretes under different loading conditions, with different types

of aggregate, and with different arrangement of reinforcement. In the meantime, it seems that use of special concretes with ultimate strain capacity

Eu below 0.003 should be subjected to specific limitations with respect to

strength and ductility, particularly where seismic loading must be considered.

B . 5 Limited Damage

While the primary function of a building code is to ensure public

safety, the designer is also interested in minimizing damage (clearly a building without any damage will protect its occupants as well!). As discussed in another Task Report in this Workshop, the code provisions recognize

the technical and economic difficulties of designing structures which could

survive any earthquake altogether without damage, and have accepted the

principle that some damage may occur in moderate and major earthquakes. It 297
Stress

Stress

Strain

-| 1 1 1 1 1 r Normal Weight

Light Weight

Compressive Strength

3 ksi

J I I L. Strain

Fig. B.ll - Test Results for Lightweight and Normalweight

Aggregate Concretes

(from Ref. 39)

298
is desirable to limit damage in moderate earthquakes to non-structural

elements, and in major earthquakes some structural as well as non-structural

damage is acceptable, provided the structure does not collapse. Furthermore,

it is desired to limit the damage so that with appropriate repairs both

servi ceabi 1 i ty (functioning under normal service conditions) and structural

integrity (ability to withstand a subsequent major earthquake) can be restored.

There is some question as to the extent that the present codes provide

for repairable damage. For example, reliance on amount of ductility which

significantly reduces the seismic design forces, necessitates acceptance of

substantial local damage due to large inelastic deformations. Depending on geometry of the frame, on the relative stiffnesses of the elements and on the

mode of the local inelastic deformation, the effective local ductility (say

for curvature in a frame girder) may be 3-5 times that of the ductility desired for lateral displacement or drift. If the latter is in the range of

4-6, than the former (curvature ductility) may have to be in the range of

10-30, with the consequence that such deformation may produce severe local

structural damage.

Development of quantitative methods for evaluating the extent of damage

and its repai rabi 1 i ty (i.e. restoration of both serviceability and integrity)

is now only in planning stages. In some cases the repair is only "cosmetic"

-- and there is very little reliable information on the adequacy of the

various repair techniques.

Certain types of damage in steel framed structures can be repaired

relatively simply by removing the damaged parts and replacing them by suitable

steel elements welded into place. However, welding in place may introduce new imperfections and residual stresses. For these reasons special care must be

exercised in quality control of such repair.

Reinforced concrete damage' may be repaired in a variety of ways. Epoxyinjection method of repair may restore original load carrying capacity, or original stiffness. However, preliminary tests indicate that hairline cracks

in concrete and damage due to slip between steel reinforcement and concrete

are not effectively repaired by epoxy injection. This tends to reduce energy

absorption capacity of repaired specimens. Also, depending on the properties

of the polymer materials used - a brittle failure may occur upon dynamic

reloading of the repaired element.

Damage and Repair . It is desirable to develop analytical methods which

would result not only in the indications of the ductility factors required

for meeting public safety objectives of the code, but would also output

indications of the type and severity of local damage.

It is also necessary to obtain basic information on repa i rab i 1 i ty

(restoration of both serviceability and integrity) of a given level of damage.

Developing such information would require an extensive series of tests. Such

a program should include development of acceptable measures of nature and

extent of damage, techniques of repair, quality control of repair, and its

rel i abi 1 i ty

.

The final decision as to the course to be followed - whether to repair

or to demolish a structure - will be based on economic considerations. Some

optimization techniques which take into account the cost of repair as well as cost of replacement are available.

299
C. Reinforced Concrete

C , 1 General

Design of concrete structures for seismic loading has been receiving

special attention for many years. Based on observations of damage in numerous earthquakes, both in U.S. and abroad, it has been suggested that concrete

structures are particularly vulnerable to earthquakes. On the other hand,

many concrete structures withstood severe earthquakes without significant

damage. This suggests that there is nothing intrinsic to concrete structures

per se to make them more vulnerable to earthquakes. Properly designed structures will perform well - whatever the material, although concrete may be

less "forgiving" or less "tolerant" of improper design. For this reason,

consideration of earthquake resistant design of concrete structures requires

special attention.

The primary requirement in earthquake-resistant design is capacity to

develop inelastic deformations (ductility) and to dissipate some of the

energy input without severe damage. In concrete structures this can be

achieved if brittle type failures such as crushing , local buckling , shear , loss of bond and anchorage are prevented, and " degradati on " ( reducti on ) in res i dual stiffness and strength are minimized. Degradation in stiffness - and loss of bond or anchorage - may be appreciably reduced by "confinement"

or "basketing" of concrete by special transverse reinforcement.

Strength and ductility of reinforced concrete elements under monotonic

axial and bending loads can be evaluated with a good degree of reliability

using well established methods. Special problems associated with ductility,

confinement of concrete, cyclic loading and reversal, buckling of steel

reinforcement, shear capacity, bond and anchorage, are discussed in the

following sections.

C.2 Ductility

Ductility is evaluated from the load-deformation relationship. Fig. B.l.

Under axial compression this requires an examination of the axial load vs. shortening or elongation curve; in flexure - it requires an examination of

moment-curvature relationship. While behavior under cyclic loading is of

prime interest for earthquake-resistant and wind-resistant design, the general

load-deformation relationships under monotonic loading gives an indication of

both strength and ductility under cyclic loading. In reinforced concrete,

cyclic loading tends to reduce the stiffness of the element, while strength

and ductility of properly designed elements need not be adversely affected.

Ductility of reinforced concrete under monotonic loading has been demonstrated for pure axial compression and for flexure. Clearly ductility of a composite, such as reinforced concrete, requires ductile behavior of both or

at least one component. Ductility of most reinforcing steels in tension is

not subject to any conditions; ductility of the steel bars in compression is

achieved when local buckling is prevented. In reinforced concrete this

requires fairly close spacing of lateral reinforcement (ties or spirals).

Plain concrete in tension behaves essentially in a brittle manner - i.e. it cracks (fractures) at a relatively low tensile stress. There is some evidence that extensibility of concrete is enhanced by proper reinforcement,

but as yet this behavior of concrete has not been generally mobilized in a practical way.

For plain ( unrei nf orced ) concrete in compression true ductility can not

be achieved, because once the maximum compression stress f^ is reached at a strain eg, the concrete can not support this level of stress with increasing

deformation. The typical descending branch of the compressive stress-strain

300
curve for concrete precludes true ductility. However, as long as the reduction in stress is gradual, i.e. the slope (df/de) of the descending branch is

reasonably flat, and as long as the ultimate crushing strain for concrete

is much greater than cq, some approximation of ductility is possible, particularly when concrete is combined with reinforcing steel.

Ductility in concrete structures is obtained when yielding of all

re i n f orcemen t- - tens i 1 e , compressive or lateral--can be developed prior to occurrence of crushing or shear failure in concrete. Flexural ductility is

primarily influenced by amounts of reinforcement in tension and in compression

p and p'; by yield strength of steel f y , and by compressive strength of con- crete fc. The effect of these parameters can be conveniently described in

terms of a generalized reinforcement index q* = [(p-p') fy/fcJ. Ductility

increases as q* decreases. Thus increasing p' and fc tends to increase

ductility; increasing p and fy tends to decrease ductility. To ensure ductile

behavior of a flexural member under monotonic loading it is usually sufficient

to limit the reinforcement index q* to about 0.75 qb*. where:

q^* = 0.85 [(EsEc^/E^e^ + f ^ ) ] = 0.72 [(87 )/( 87 + f ^ ) ]

is based on assuming that yielding in tension steel and crushing of concrete

at a compression strain 0.003 occur simultaneously ("balanced condition").

When plastic hinges develop in girders and when the redistribution of moments

is used in design, only 0.5 of the reinforcement ratio producing balanced

conditions is allowed in order to prevent premature crushing. For seismic

loading conditions the 0.5 "balanced" ratio limitation is also specified in

the ACI Code. When p approximately equals p' (for example in beams where

seismic effects are much greater than gravity effects, so that practically

full reversal of moments can be expected at the crtical section) - the

limitation is satisfied whatever the value of q. However, it may be desirable

to limit the amount of tension reinforcement so that (p f /f') < in order

topreventortolimitcrushing.

In general, as f^ increases the value of q* decreases and leads to an increase in ductility. This conclusion must be accepted with reservati ons --

as some concretes tend to behave in a more brittle manner as strength in- creases [39]. Brittleness of some concretes may be associated with aggregate

weakness and therefore a general conclusion on improvement of ductility with

high strength concrete does not seem appropriate.

Typical reinforced concrete elements with conventional ties fail in a rather brittle manner when subjected to axial compression. The mechanism of

failure is due to crushing of concrete cover and exposing the longitudinal

steel, which then buckles and is followed by failure of the central portion

of concrete. To achieve a more ductile mode of failure Considere [77]

demonstrated the effectiveness of spirally reinforced concrete columns for

resisting axial compression. He discussed the increased capacity and ductility of confined concrete and practical details of construction to achieve

effective confinement. Brandtzaeg, Ri chart, and others [83] obtained additional evidence on effectiveness of spirally reinforced concrete columns

under axial compression. Recent tests and experience have demonstrated

excellent performance of spirally reinforced concrete columns; however, their

effectiveness under reversed cyclic lateral loads has not been fully evaluated

[12].

The strength of a reinforced concrete element under compression and

bending is usually described by a P-M interaction diagram, such as shown in

Fig. C.l .

Axial compression also influences moment-curvature relationship. Because

axial compression postpones initiation of flexural cracking, the initial slope

of M-(j) curve remains steep up to a larger value of moment. Also increase in

axial compression up to P = Pj^ increases moment capacity from to .

301
302
However, even in this range 0<P<Pb, the influence of axial load is to produce

crushing at lower curvature than for pure bending. With further increase in

P, crushing occurs at lower curvature values, and the effect is to produce a more brittle mode of failure, that is to reduce the ductility which can be

attained under pure flexure. Fig. C.2.

C . 3 Conf i nement

When concrete is confined laterally ( hydros tati cal 1 y - by a metal jacket,

or by a closely spaced circular spiral) it can withstand large compressive

strain as well as large compressive stress, and can be characterized as a "ductile" material. However, most recent studies of concrete confined by

hoops and rectangular "spiral" indicate that these are not fully effective

[2]. Typical results are shown in Fig. C.3. If confinement comes into play

before concrete reaches its maximum stress, then the stress increment Afj- may

be substantial, but as the hoops reach their yield strength they cannot

provide additional confinement, and the stress-strain curve follows a descending branch similar to that of plain concrete. In this case, ductility of

confined concrete is not reliable. When confinement is "delayed", its con- tribution maintains the ductility - but the strength increment is small. The

"delay" in confinement can only be achieved when the concrete has a low value

of Poisson ratio initially, and its increase starts at a high level of com- pression stress, as close as possible to compressive strength f^.

Confining effectiveness of lateral reinforcement depends on its shape.

As shown in Fig. C.4 the effectively confined zone ("core") is much smaller

for columns with square single hoops than it is for a spiral column. Assuming full confinement (i.e., additional strength of concrete due to confinement is taken as 4x confining pressure) the required spacing s of spirals or hoops is

J

s = 2.(f^/f;)(lh) /A 1-lJ

c

^

where d, - is the diameter of spiral or hoop wire, - is core diameter,

Ag - gross area, and Ac - effective core area. Typical required spacing s for hoops and spirals is shown in Table C.4.

Table C.4 HOOP ANU SPIRAL REQUIREMENTS BASED ON CONFINEMENT OF CONCRETE

Column size 18 x 18 in. square

Concrete cover 1.5 in.

Spiral steel fy=60ksi

Tie steel fy = 40 ksi

Concrete Strength 3 ksi 6 ksi

Hoops

Size No. 2 No. 3 No . 2 No. 3 Spacing, in. 3/8 7/8 1/4 3/8

Spirals

S i ze No. 2 No. 3 No. 2 No. 3 Pitch, in. 3/4 1 3/8 3/8 3/4

It can be seen from the table that when the strength of concrete is high

and yield strength of hoop steel is kept to grade 40, the required spacing

303
Strain

Fig. C.3 Effect of Different Methods of Confinement

on Stress-Strain Characteristics of Concrete

in Compression,

(from Ref. 2)

304
Fig. C.4 Effect of Hoop Shape on Confinement

305
may equal hoop wire diameter! Clearly, hoops can not be spaced so closely,

which simply means that adequate confinement can not be developed by a single

hoop

.

Present (1971) SEAOC design recommendations provide for use of multiple

ties assigning them an effectiveness somewhat less (about 20% less) than that

of a spiral. Tliis assumption has not been substantiated by tests. The SEAOC

provision allows for a maximum hoop spacing of 4 in., and it is not clear that

an effective confinement can be provided with such spacing even with multiple

hoops.

Two other factors tend to limit effectiveness of hoops. The intensity

of the confining stress depends on the hoop tension which is reacted

locally by bearing stress 05^ on the concrete core. For ci rcul ar hoops , Fig. C.5 this stress 05^ can be expressed in terms of : %r - (2a,^d,2/4d,.D^) = 1.57 a,(d,/Dj

Depending on concrete characteristics and the ratio of hoop wire diameter d. to core diameter D^- this stress may limit hoop effectiveness. For example,

if tlie bearing stress is limited to 3 ksi (as for some lightweight aggregate

concretes) and for = 12 in., dj^ = 0.375 , the limiting hoop stress will be

a, = a, (D /I .57d. ) =61.2 ksi

h b r

^ c h

In this case the effectiveness of high-strength hoop wire cannot be fully

realized. The following table shows maximum values of for 3/8 in. and 1/2

in. diameter steel hoops for two types of concrete having bearing strengths

0[jp of 3 and 5 ksi, and three columns sizes having core diameters of 10, 15

and 20 i nches .

Table C.2 HOOP STRESS LIMIT VS. BEARING STRESS

(Hoop Stresses Shown in Ksi.)

3 Ksi 5 Ksi

D^-in """"-^-^^ 3/8 1/2 3/8 1/2

10 51 38 85 63.5

15 76.5 57 127 95

20 102 76 170 120

It can be seen that for concretes which cannot withstand a high local bearing

without crushing, effective hoop stress intensity may be limited to 40-70

ksi, particularly for concrete columns with relatively small core diameters.

Hoops prevent bulging of the core locally--but depending on the spacing

of the hoops some bulging may take place in the interval between the hoops.

This action produces some local transverse cracking in the zone near the hoop

wliich reduces tlie effective core area. The cracking is shown in Fig. C.5.

This cracking depends on hoop spacing and concrete characteristics

306
(particularly its tensile strength) and causes a reduction in the core area which may reduce the effectiveness of confinement and, in extreme cases, may

greatly reduce the ductility of the column. The depth of the crack c may be

related to hoop spacing, so that

c = as

and the diameter D of the effective core area is then

e

D = D -2c = D (1-2k)

e c c where k is (as/D ). The area of the effective core is A and its ratio to

the normal core Srea A are:

^

c Ag = (TTDg2/4) = (uD^^/4)(1-2k)2 = BA^

3 = (Ag/A^) = (Dg/Dj^ = 1 - 4k(1-k)

It is clear that k cannot exceed 1/2, which correponds to the stage when

transverse cracks have propagated through the entire core, i.e., the column

no longer is in compression. The core area reduction factor 3 calculated

for a given value of k is shown in Table C.3.

Table C.3 CORE AREA REDUCTION FACTOR 6 VS. CRACK DEPTH FACTOR

K = ots/D^ 0.01 0.03 0.06 0.10 0.15 0.20 0.25 0. 30

3 = A^/A^ 0.96 0.88 0 . 78 0.64 0.41 0.36 0.25 0.16

It can be seen that peripheral cracks extending only 3% of the diameter into

the core reduce the effective core area by 12%.

In the absence of effective "confinement", advantages may be derived

from multiple rectangular hoops as a consequence of "binding" effect (sometimes referred to in European literature as "stitching through" effect, or

in American literature as "basketing" effect). Also use the multiple

rectangular hoop (ties) provides necessary restraint of longitudinal reinforcement against local buckling of reinforcing steel and in this way plays

an important role in developing full strength of compression reinforcement.

C . 4 Cyclic Loading and Reversal

Studies on singly-and doub ly- re i n forced concrete beams under cyclic

loading without reversal [28, 29] have shown that there is no reduction in

strength with cycling. Fig. C.6, when amount of tension reinforcement is

below (pfy/fc) = 0.25, and when adequate shear reinforcement and hoops

(stirrups ) to prevent local buckling of compression steel after crushing

of concrete are provided.

Studies [31] on beams under cyclic loading with reversals indicate that

while properly detailed beams can develop their full strength (as determined

for static loading) there is a reduction in initial stiffness at successive

307
Fig. C.5 Confinement Using Circular Hoops

c c

or Spirals

Moinent

- Curvature

Fig. C.6 Moment Curvature Relationship

of Reinforced Concrete Beams

Under Cyclic Loading

(from Ref. 29)

308
cycles in which load or deformation is increased beyond that in any previous

cycle. The energy absorption and energy dissipation capacities are also

sensitive to the loading history. While el asto-pl asti c idealization may

overestimate energy dissipation capacity by as much as 100%, the capacity for

energy dissipation may be subs tan ti al -- i ncreas i ng with the amount of inelastic

deformation (compare energy dissipation in cycle 20 with that in cycle 32 as

s hown in Fig. C . 7 )

.

The factors which influence the shape of the hysteresis loop for a rectangular doubly-reinforced concrete beam are the following: (a) inelastic

deformation of steel reinforcement - taking into account alternating plasticity, (b) extent of cracking in concrete - taking into account incremental

nature of crack width, (c) effectiveness (or deterioration) of bond between

reinforcing steel and concrete - including the possibility of slip (or partial

loss of anchorage) at some distance from section being considered, and (d)

presence of shear (diagonal) cracking or shear deformation.

The process of local deformation with large load (or inelastic strain)

reversals. Fig. C.8 has been described in reference 31.

In a doubly-reinforced concrete member loaded well into the inelastic

range the major flexural crack denoted as in Fig. C.8.1, will not close

completely on unloading. Fig. C.8. 2. The degree of opening will depend on how

far the tensile steel was strained into the plastic range during first loading.

If the tensile steel was strained well beyond the initial yielding, a crack

C2 may originate on the bottom side during unloading.

If the member is then loaded in the opposite direction, the critical

section which has already been cracked will offer considerably less resistance

to rotation than during the first loading. This decrease in resistance may

be caused by the fact that two faces of the former crack are not in perfect

contact. The crack at the top may close or not, depending on the peak value

of the reversed load relative to P-j on the amount of top and bottom reinforcing steel, and on other factors. Because the concrete in the two faces

has undergone a process of disruption, a reduction in the stiffness of the

critical region should occur even if the crack closes.

If the load in the reversed direction reaches the same peak value as

b t P-j, the width of the crack would be larger than -- observed under P^.

It is evident that if the member is now unloaded, the critical cross-section

t b will be cracked throughout - and the width of the crack will depend

mainly upon the amount of yielding of the steel, the effectiveness of composite action (bond) between steel and concrete and, to a lesser extent, on the

degree of the concrete disruption.

At the start of a new cycle of alternating load, the original doublyreinforced concrete section will behave as steel cross section represented by

the tensile and compressive steel reinforcement. If the reinforcing steel

exhibits pronounced Bauschinger effect, this will result in the reduction of

stiffness of the critical region. Furthermore, at this stage, the presence

of shear will tend to displace the faces on either side of the crack relative

to one another. Fig. C.8. 5. The tendency is resisted by the "dowel action"

of the main reinforcement and will cause the steel bars to be pressed against

the concrete and consequently may lead to longitudinal splitting of the con- crete. The degree of damage introduced by this effect of shear will depend

309
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310
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31

1
on tie spacing, but it is most likely that it will affect the bond and con- sequently the overall stiffness of the member.

Deterioration of bond which may be increased by the shear in sections

cracked throughout may cause local failure at any point of discontinuity in

the main reinforcement and particularly in the beam-to-exterior column joint.

For example, in the joint shown in Fig. C.8.6, deterioration of bond along

length BA due to alternating stress in the steel and the effect of any shear

force acting in section C'- - C may lead to the development of high radial

stresses at A and a bearing failure may occur. This may lead to spalling of

the concrete beneath the bar, particularly if the amount of concrete cover is

inadequate. This type of failure has been observed in several investigations

and in field inspections of earthquake damage.

The behavior of the critical region at the girder-column joint may be

affected both by the shear and also by the details of anchoring the girder

reinforcement. In particular, steel detailing in a monolithic connection

between the exterior column and the girder may contribute to stiffness

deterioration.

Precise evaluation of the stiffness is not possible at present because

of the absence of adequate basic experimental information. However, it is

clear that once a sufficient number of cycles occur, bond between cracks may

completely disappear and, therefore the initial stiffness (at M = 0) will be

provided by the steel reinforcing alone, where concrete acts just as a filler

and consequently any stiffness deterioration from then on will be due to more pronounced Bauschinger effect as the inelastic deformation increases. Thus,

the stiffness degradation of inelastic regions subjected to pure bending

reverslas is a consequence mainly of bond deterioration and Bauschinger effect.

While repeated reversal loadings have detrimental effects on the initial

stiffness of reinforced concrete members -- due to the yielding of steel and

bond deterioration between cracks --, they may have beneficial effects on the

ductility of such members. In the case of a reinforced concrete region subjected to monotoni cal 1 y increasing bending moment, sudden unloading may occur

due to crushing of the outer concrete cover and buckling of compression steel,

as illustrated in Fig. C.9. However, under repeated reversed loading, as a result of the gradual opening of cracks due to the yielding of steel, the

sudden (brittle) failure of concrete is delayed because the concrete is not

strained in compression until the cracks have completely closed, i.e., the

section is converted from conventional reinforced concrete, in which ductility

is controlled by the semi-brittle concrete, to essentially a steel section in

which the compression steel is restrained against buckling by the concrete

cover and therefore provides high ductility, Fig. C.IO.

C . 5 Buckling of Reinforcing Steel

In reinforced concrete elements (beams and columns) there are a number

of reasons for spalling of concrete cover: crushing at a high compression

strain, concentration of local stresses and strains at transverse steel hoops,

high shear and bond stresses (splitting preceding spalling), etc. Compression

of reinforcing steel, which is not confined by the concrete or properly re- strained (braced) by lateral ties (hoops or stirrups), will lead to local

buckling, Fig. C.ll. This problem was treated in Ref. 76 for static monotonic

loading. It was suggested that spacing of ties s (stirrups) can be derived

from the condition that the critical inelastic buckling stress shall not be

less than the yield strength of reinforcing steel. If D is the diameter of

the longitudinal reinforcement then

(s/D) = B(E^/F^)^/2

where is tangent modulus at yield strength based on non-linear stress31 2
Moment

Fig. C.9 Reinforced Concrete Beam Under Monotonic Loading,

(from Ref. 31)

Fig. C.IO Reinforced Concrete Beam Under Cyclic Reversed Loadlnp,

(from Ref. 31)

313
mm

m

F = o2e (r/s)2 = F

cr t y

s = 7r/c(E^/Fy)'^(D/4)

Fig. C.ll Buckling of Reinforcing Steel Exposed

After Spalling of Concrete Cover

(from Ref. 76)

314
strain diagram (el asto-pl asti c idealization is not sufficient for this

1/2 representation), and B = O.SttC ' = 2.2. For typical values of F and E. = y t 10,000 ksi values of s vary from 16D to 12D, the closer spacing corresponding

to Fy = 85 ksi.

If the compression steel reinforcement is expected to withstand stresses

(strains) in the strain hardening range, then the values of should be

chosen accordingly. Estimating E^ = 1,600 ksi, the spacing of stirrups (ties)

should not exceed 6D (or even 5D for higher values of F^).

Under reversed loading a value of E^ = 2,000 ksi may be appropriate for

reinforcing steels having F^ = 50 ksi. The tie spacing s under these conditions would vary from 7D to 5D.

It should be noted that the 1971 reversion of the SEAOC recommendations

calls for a maximum spacing s not greater than 80. This requirement is

clearly justified for cases when reversal is present. For cases of monotonic

loading the appropriate value of s depends on the extent of inelastic deformations of reinforcing steel bars. If it extends into strain hardening range,

then the value of s should not exceed 5 to 6D, as shown above. However, where

no reversal occurs and where the inelastic deformation in the elements is also

limited - then the tie spacing may be increased by about 50%.

In providing lateral restraint against buckling three factors are of

importance: (a) size of hoop (or ties), (b) effective restraint of the

longitudinal bar (absence of any freedom of movement because of the flexibility or gaps between ties and main bars), (c) adequate anchorage of hoop ends

(prevention of "opening up" of hoop or tie).

The size of the hoop is important as it affects its strength and stiffness. An analytical evaluation of required tie stiffness [76] showed that

when restraint was provided by extensional stiffness (ties bend around main

bar) rather than flexural stiffness, the required tie size was quite small.

For such shapes the size should be selected to prevent premature local crushing in concrete or fracture in the tie restraining the lateral deformation of

concrete

.

To provide effective restraint the ties should be bent around the bar

(limiting the included angle to somewhere between 90-135°), should be in

contact with the bar, and attached to it by tie wire. While under monotonic

loading a gap between the bar and the tie may not be important, as it is

likely to be filled by concrete, under cyclic reversal of loading any concrete

between the tie and the bar will be destroyed. With successive reversals of

tension and compression in the main steel reinforcement and the surrounding

concrete the gap will allow lateral movement of the bar, thus aggravating

initiation of buckling. Reasonable tolerances of tie placement with respect

to main bars, which will minimize possible buckling of main bars require

further study.

To prevent "opening-up" of hoops through anchorage failures, it is

advisable to bend each end through a 135° angle with a suitable straight

extension (about 10 bar diameters) in a region of concrete which is preferrably subjected to compression. Hairpin ties or caps may not be fully

reliable and their effectiveness should be investigated experimentally.

C .6 Shear

High intensity shearing stresses in reinforced concrete elements produce

inclined ("diagonal tension") cracks which, if allowed to propagate may cause

31 5
failure by one or more of the following modes: (1) stress concentration at

the tip of the crack in the compression zone and subsequent crushing, and

(2) dowel action of steel reinforcement producing secondary longitudinal

splitting along the bars.

The first mode may lead to buckling of compression steel after crushing;

the second - leads to loss of bond, and may result in failure due to loss of

anchorage. Even in the absence of failure - loss of bond results in substantial reduction in stiffness.

Shear failures can be prevented by arresting the propagation of inclined

cracking by adequate transverse reinforcement (ties or stirrups) which should

limit diagonal cracking and prevent crushing or splitting due to shear. Also

the transverse reinforcement should be sufficient to resist a portion or all

of the shear on the section.

The magnitude of the shearing force V for which the element must be

designed should be at least equal to or greater than either:

or

= [(MpT + Mp2)/h] + 0.5 wh

where and are shear forces due to factored dead and earthquake loads

respectively, and Mp2 are plastic hinge moments at the ends of an element

or its segment, h is the distance between the plastic hinges, and w is the

intensity of uniformly distributed load on the element.

The total shear capacity of a reinforced concrete element usually has

th ree components

:

V = V + V + V , c sw si

concrete shear resistance (if any),

transverse (web) steel shear resistance developed by tension

instirrups(ties),

V, = longitudinal steel shear resistance developed by dowel action

of main bars.

The concrete component is determined by the shear capacity of the uncracked

section, with some contribution from the aggregate interlock shear resistance

(frictional resistance). Normally, the shear strength of the uncracked portion of concrete and the aggregate interlock are enhanced by compression up

to a limit stress of about 0.8 f.c For reinforced concrete elements under monotonic loading the value of

may be taken as:

= 4.( 1 + 0.0005 N^/Ag)(2/f^)(bd)

where - design axial load occurring simultaneously with shear Vu , positive

for compression, negative for tension, and Ag is gross area of the section.

The shear resisted by the transverse steel reinforcement may be approximated by the conventional assumptions of equal participation of all stirrups

where V

c

^sw

316
(ties) in a projected length of a crack inclined 45° to the axis, in which

case the number of stirrups resisting the shear is (d/s). Then

V = f . A . (d/s)

sw SV V

Under monotonic loading, when failure occurs at a section critical in shear,

and all the effective stirrups are assumed to yield:

V,,, = f . A . (d/s)

sw y V

\ ' '

The shear resisted by the longitudinal steel is developed by dowel action

of steel bars. Effectiveness of such action depends greatly on preventing

splitting along longitudinal reinforcement (i.e., on spacing of stirrups or ties). Usually contribution of longitudinal steel shear resistance is ne- glected (i.e., = 0). This is a conservative procedure - which is particularly desirable because the estimate of V based on f = f may be somewhat

unconservati ve . ^

In the absence of any reversal of shear and moment but with cyclic inelastic deformations the shear capacity of concrete may be reduced: diagonal

cracks may propagate further into compression zone, local bond may deteriorate,

diagonal cracks may become wider and not close after the disturbance. Thus,

the contribution of concrete to shear resistance may be reduced to about 1/2

that normally assumed as effective.

Under reversed loading diagonal cracks develop in both directions forming

typical X-shaped cracks. In this region the shear must be carried by the

stirrups with some contribution by the dowel action. The magnitude of shear

carried by the dowel action is highly indeterminate, as it is only possible

if longitudinal splitting along the main bars is prevented by closely spaced

stirrups. No shear whatever can be transmitted by the concrete. Even

"aggregate interlock" is unreliable as it is destroyed by loosening and

fracturing of the aggregate with cyclic reversals of loading and "working

back and forth" along the diagonal cracks. Therefore, in beams where full

reversal takes place, assuming and Vg] are zero, the stirrups must be

proportional for the entire shear, i.e., = 0, and

» V.s " A f d

Effects of detailing special transverse reinforcement in the area of

reinforcing bar cut-offs and splices on behavior under cyclic repeated loads

have not been studied extensively. Therefore, provisions in AC I 1971 Code

Sections 12.1.6.1, 12.1.6.2, 12.1.6.3, and 7.12.6 are to be considered minimal .

C . 7 Combined Axial Compression, Bending, and Shear in Columns

In ductile moment-resistant reinforced concrete frames the columns are subjected to axial loads (primarily compression due to gravity) and to reversals of shears and moments due to lateral loads (seismic, wind, etc.). Under

reversed shear and moment loading, with relatively constant compression, the

main effects to guard against are reduction in shear capacity (due to diagonal

cracking) and buckling of compression steel (after spalling of cover). The

reduction in shear and buckling of compression steel are more critical in the

presence of reversal

.

Two possible cases are shown in Fig. C.12. It can be seen that the

values of maximum moment are about the same for the two cases. However,

initial loading conditions are quite different, and while almost complete

317
max

/ /

tr±3

F«

Column with

Reversal and/or

S t rain-Hardening

hi

rt-I

Column Without

Reversal and /or

Strain-Hardening

Fig. C.12 Effect of Reversal on Spacing of Hoops or Ties.

318
moment reversal occurs in one case, there is no reversal in the second case.

It seems reasonable to expect that in the case with reversal and/or with

strain hardening the spacing of ties should be selected more conservatively

than in the case without reversal and/or strain hardening.

Resistance of reinforced concrete columns to repeated alternating shear

and bending, simulating seismic loading in columns of moment resisting frames

has been studied experimentally by T. Hisada and his associates [12].

Dimensions of specimens, arrangement of reinforcement, and test set-up

are shown in Fig. C.13. Reduction in the resistance of the columns at a specified deflection with alternating loading cycles can be expressed by a "reduction factor" A which for any given cycle is defined as follows:

The reduction factors for the nine specimens are shown in Fig. C.14. It can

be seen that for spirally reinforced col umns--reducti on factor reaches a limit value of about 0.70 at about 30 cycles. Most of the reduction in the

resistance takes place in the first 10 cycles. The limit reduction, is not

sensitive to the amount of transverse reinforcement. For columns with ties

restraining each longitudinal bar by a 90° bend, reduction factor at 20 cycles

varies from 0.62 to 0.84 and is sensitive to the amount of transverse reinforcement. It is interesting to note that such tied columns exhibited a lesser reduction in resistance than spirally reinforced ones. This may be

due partly to a different arrangement of longitudinal bars, and partly to a more effective restraint of longitudinal bars provided by a 90° bend in the

tie as compared to a circular hoop of a spiral.

Columns with rectangular hoops, which restrain corner bars only .performed

poorly, exhibiting the greatest reduction in resistance. After 25 to 30

cycles the resistance was reduced to about 25-30%.

The results of these tests clearly indicate that properly designed

tied columns perform quite well under combined axial compression, and

reversals of shear and bending. Furthermore, these tests suggest that in some cases SEAOC requirements for lateral reinforcement, based on the concept

of "confinement" may be too conservative.

Several special problems may arise in column design which usually do

not arise with regard to beams. One of these is the skew loading (not in

plane of symmetry). The plane of the lateral forces may vary in direction

during the cyclic variations in load and it has been suggested that the

capacity of columns may be sensitive to the variations in the loading plane

as well as to the variations in the load magnitude.

Another problem is the variety of column shapes, e.g. L- or U- shaped

column sections. This problem is related to the skew loading - and there is no experimental evidence on behavior of such irregularly shaped columns

under cyclic reversals of shear and moment.

C . 8 Connections in Moment Resistant Frames

One of the critical areas in the design of ductile moment-resistant

frames is the joint between columns and beams, Fig. C.15. Under gravity

loading the end moments in the adjacent beams tend to cancel each other, and

only the moment unbalance, which must be resisted by the columns, induces

shear in the joint. Under seismic conditions the end moments in the adjacent

beams act in the same sense, and the columns must resist the sum of beam

moments. The transformation of beam moments into column moments generates

large shears in the joint, and repeated reversals of moments (shears) may

319
Hooped

Fig. C.13 Dimensions of Specimens and Test Set Up

(from Ref. 12)

320
100 X, Shear Capacity,

80

60

40

Jl

^J3

20

10 20 30

Number of cycles

Note - Transverse reinforcement significantly below

that required by SEAOC Recommendations

.

Fig. C.14 Reduction in Shear Capacity at a Specified Deflection

(from Ref. 12)

321
Gravity Loads Lateral Loads

Pig. C.15 Gravity and Lateral Loads on a Joint

322
produce extensive cracking and loss of bond between concrete and reinforcing

steel .

The first loading cycle, Fig. C.16, produces cracks in beams and

columns and yielding of reinforcing steel in these regions leads to fairly

wide cracks. Upon reversal the first cracks close and cracking and yielding

develops on the opposite face.

The stress distributions on the faces of the joint region and the

freebody of the top bar are shown in Fig. C.17. It may be assumed that the

tension stress is equal to yield strength fy , and that the compression stress

f' is less than fy . Then while the concrete around the bar is still intact,

bond stresses are developed along the full length. The distribution of the

bond stresses depends partly on the state of stress of concrete surrounding

the bar - where concrete is in biaxial compression, the resistance to slip is

greater and the bond stress intensity is greater. Where concrete is in tension, resistance to slip is diminished and the bond is reduced.

With cycling, there is likely to be considerable deterioration of bond

or slip resistance. Based on experimental results [26, 69] the following

mechanism of bond deterioration under repeated loads was suggested:

1. The basic mechanism of bond deterioration is a failure in the

concrete "boundary layer" adjacent to the steel -concrete interface. This failure occurs when the high local stresses reach

critical values, and inelastic deformation and/or local

fracture take place. Due to the nature-of the failure surface

and to the interlocking of the deformed bar lugs with with

surrounding concrete, shearing stresses below the critical

value may be transmitted by friction and by wedging action.

Some slippage of the steel relative to concrete takes place

and is accompanied by inelastic deformation and local crushing

at the steel -concrete interface.

2. With repeated cycles of loading and unloading, provided the

tension in the steel does not exceed the previous maximum, there

may be some further disruption in the boundary layer.

3. The effectiveness of bond between concrete and steel depends on the given level of tension stress in the steel and on the

magnitude of the previous maximum tension. The greater the

magnitude of the previous maximum tension, the greater is the

disruption of the boundary layer, and the lesser is the

effectiveness of the bond at lower levels of tension. When the

previous maximum tension is considerably greater than the working

stress level, even a small number of cycles of this high tension

reduces the bond effectiveness at the working stress level.

With repeated cycles of reversals in the inelastic range, longitudinal

splitting takes place in the joint further reducing bond effectiveness and

allowing considerable slip (and hence rotation) in the joint. Finally,

resistance to slip is generated only in the compression zone, where "bond"

is effected only by friction and wedging.

After several extreme strain reversals the joint can act only as a mechanism with flexible joints. This behavior can greatly increase the

lateral displacements, and thus increase the lateral ductility. On the

other hand local damage will be greatly increased and possibility of frame

and member instability enhanced. Furthermore, the joint moment resistance

will be decreased, and the collapse mechanism will be altered, reducing the

load carrying capacity of the frame as a v/hole.

The deterioration of the joint effectiveness described above is based

on test results with planar frame subassemblies. It has been shown that for

323
Cracking on First Loading and Reversal

324
f

s

Dili

Top bar

m

N ;

!

1

1

1

1

I Bond stress

After Cycling

C.17 Stresses on Joint Faces and

325
three-dimensional frame joints - presence of transverse beam stubs considerably improves the performance of the joint, [55J. Presence of a floor slab

should further improve joint behavior, unless the transverse beam elements

and the slab are also subjected to reversals of inelastic deformations

(loading) .

C.9 Shear Walls

The ability of reinforced concrete walls to resist large shear forces

with small deformations makes them particularly effective structural elements

in earthquake resistant design. It must be recognized however that their

rigidity while limiting deformation (drift), also changes the response of

the structure: it reduces the period, increases the shear forces generated,

and increases the number of load reversals. Also, unless the walls are properly designed, they may fail in a rather brittle mode - greatly reducing

their potential energy dissipation capacity. Such failures are also difficult to repair fully, i.e., to restore the structural integrity which would

ensure their effectiveness in a subsequent earthquake.

A number of difficulties arise in developing proper design criteria for

reinforced concrete walls. In this review the discussion will be limited to

shear wal 1

s

, which can be characterized by the fact that the wall panels act

primarily as shear elements in a frame which resists bending moments primarily

by axial forces. This implies that shear walls constitute panels (in-fill)

with frame members on all four edges.

The desired behavior of the system is to have a ductile frame sufficiently strong and properly reinforced so as to restrain excessive distortion

of the wall after initial cracking, and to produce a large number of small,

well distributed, cracks in the wall panel. In this way maximum energy

dissipation would be achieved, a'nd at the same time overall lateral deformation would be relatively small, and the frame would undergo minimum damage.

The optimum shear wall design is to produce a combination of a ductile frame

with a ductile wall panel. This effect has been achieved in some cases by

such devices as a "slit shear wall" designed by Muto [79].

It has been suggested that shear walls will crack initially when they

reach a shearing strain of 0.2 to 0.3 x 10"3 radians, and usually the shear

cracks originate at a stress substantially higher than that for reinforced

concrete beams (two to three times as great).

The shear capacity of a reinforced framed wall depends on the amount of

reinforcement and on the frame contribution. Because the ultimate objective

of a shear wall is to allow formation of a large number of diagonal cracks,

it may be appropriate, from the strength point of view, to disregard the

contribution of the concrete to shear strength of the wall panel. The shear

capacity would be defined by the amount of reinforcement and by the shear

capacity of the frame surrounding the panel. This criterion might provide

a basis for frame design - frame should carry the excess shear (design total

shear less that carried by wall reinforcement). Of course other requirements

for proportioning frame members (compression, flexure, ties for preventing

local buckling, etc.) must be taken into account.

The shear wall system described above must be differentiated from a "deep beam wall", which does not have an adequate frame to resist bending

moments and to constrain the wall in such a way that it would generate a large number of small cracks. In such "beam wall" systems, often one major

crack forms suddenly and greatly reduces the capacity of the wall to resist

lateral forces. A "beam wall" or "frameless wall" must be designed to resist

the lateral shear without cracking.

In design of shear walls the following questions must be answered: (1)

326
what is the load (or stress) that causes first cracking in the wall, (2) what

is the capacity of the wall (acting with the frame) to resist shear, (3) what

are the proper criteria for design of a suitable ductile frame to act with

the shear wall, (4) what are the proper criteria for detailing wall and

frame reinforcement and dimensions, and (5) what are the proper provisions

for design of shear walls with openings.

A major problem in design of shear walls is the effect of openings. The

number and size of openings must not be too great; otherwise the behavior of

the wall will not be realized, for it can become merely a frame with some

deep spandrels and short columns. Assuming that the openings are relatively

small (with respect to the size of the wall panel) the usual criteria in the

U.S. Codes is to provide a specified amount of corner and edge reinforcing

bars

.

It is suggested here that a greatly superior system of reinforcing

around wall opening would be obtained by creating an effective frame of

reinforcing steel around the opening as shown in Fig. C.18. The basis of

this design would be to strengthen the subpanels A, B, and C surrounding the

opening in such a way that the wall with the opening would have the same rigidity and capacity as a wall without the opening.

At the present time there is no sufficient information on which

rational design criteria could be based. Therefore, additional studies - both analytical and experimental, are required. Particularly important would

be studies of interaction between frame and panel elements in shear walls,

failure mechanisms, capacity, ductility, and energy dissipation - particularly

under cyclic reversed loadings.

D. Prestressed Concrete

D . 1 Prestressed and Partially Prestressed Elements

Prestressed concrete structures in localities where severe earthquakes

and winds are anticipated have been used only to a limited extent, although

recently use of such structures has been increasing. Published experimental

studies of behavior of prestressed concrete elements under cyclic reversed

loadings also have been limited. In a review of these studies Bertero [90]

observed that prestressed concrete framed structures can resist moderate

earthquakes without structural damage, and can withstand severe earthquakes - although in this case, structural damage may occur with consequent difficulty

of repair restoring structural integrity of a fully prestressed system.

Some of the difficulties experienced in prestressed concrete structures

in seismic regions is related to connection details of precast elements. In many cases the prestressed elements themselves behaved well. Test of such

elements under monotonic loading revealed amply ductile behavior (large

energy absorption), and largely elastic recovery (little energy dissipation,

low damping). Partially prestressed concrete members (containing some con- ventional reinforcing steel) have greater capability for energy dissipation.

In the absence of sufficient data, no recommendations can be made as to

revised design procedures of prestressed concrete systems. In carrying out

such designs however, considerable attention must be devoted to consequences

of local crushing or cracking in critical regions of the elements and connections, caused by inelastic cycling caused by extreme loads.

Additional research is needed to determine behavior of realistic models

(subassemblies) of prestressed and partially prestressed concrete systems

under cyclic reversal of loads in the inelastic range.

327
Fig. C.18 Reinforcement of Wall Openings

328
E. Structural Steel

E . 1 General

The viewpoint that steel structures are less vulnerable to severe earthquakes than masonry or concrete structures because of the natural ductility

of the material has some merit. However, observations of damage in severe earthquakes can not fully support this point of view, first - because some

steel structures have undergone damage in severe earthquakes, and second - because many severe earthquakes occurred in areas where only a few steel

structures existed, thus biasing (statistically) occurrence of damage in

structures of different materials. At best, considering the natural ductility

of steel, one can conclude that steel structures tolerate design or construction inadequacies more than other types of structures.

Basically, the requirements in earthquake resistant design of steel

structures is the same as for all structures - i.e. ductility and energy

dissipation capacity without excessive cumulative damage. Any conditions

that tend to counter ductility, energy dissipation and endurance of load

reversals without damage must be closely examined. For steel structures

these are: frame deformation (P-A) effects, local buckling, and problems

associated with connections (welded or bolted).

While modern design criteria for concrete structures are primarily based

on strength and overloads, current design criteria for steel structures are primarily based on allowable stresses and service (working) loads.

Section 1.5.6 of AISC Specification for the Design of Structural Steel

for Buildings states that under wind and seismic conditions allowable stresses

may be increased one-third above the values under normal service conditions.

Depending on the level of (D+L) load stresses, this criterion may lead to a wide range in the reserve capacity for overloads. The inconsistencies in reserve capacities are best illustrated by simple examples suggested by

Professor T. Y. Lin (private communication).

Consider a ductile steel element for which the strength may be characterized by plastic capacity Py (also yield capacity). The allowable load

under normal service conditions is Pa = 0.6 Py , and under combined (D + L+E)

conditions it is Pg = 1.33 Pg = 0.8 Py . Should the design earthquake intensity be underestimated, it is expected that the reserve capacity P^ = ( Py - 0.8 Py) = 0.2 Py will accommodate the increase in earthquake intensity At.

The factor X is the ratio of reserve capacity P^- to earthquake effect P^, i.e.

X = '^r'^^E^"

T'^'is factor will vary with the levels of (D + L) and (E) stresses.

If (D+L) condition produces a design load P] equal to 0.6 Py , and if

design earthquake effect due to E is Pei = (Pa'^l) ~ ^-^ Py (gravity load

controls design, earthquake effect is small), then A = (P^-ZPe) = (0.2 Py/0.2

Py) = K or a 100% increase in earthquake effect can be accomodated. This

condition may be typical of girders, particularly in upper stories, where

earthquake effect is small compared to gravity. On the other hand, if (D+L)

condition produces a design load P2 = 0.2 Py , and if design earthquake effect

PE2 = (Pa " P2) " 0.6 Py (i.e. gravity effect is small and earthquake effect

controls the design), then X = (P^/Pe) = (0.2 Py) = 0.33, or only a 33%

increase in earthquake effect can be accomodated. This condition may be

typical of columns in a ductile moment resistant frame, particularly in lower

stories.

It can be seen from the above that a fixed increase in allowable stress

(say 1/3) can not give a uniform A reserve capacity for earthquake loadings

for all elements in the structure.

Another inconsistency appears to exist in the AISC provisions, which

stems from possible differences in the interpretation of "low cycle fatigue"

effects. AISC Specification, Section 1.7.1, states that "the occurrence of

329
full design wind or earthquake loads is too infrequent to warrant consideration in fatigue design." On the other hand under provisions of Sec. 1.7.2

for some types of welded connections, e.g. fillet welds and transverse partial

penetration groove welds, subjected to a "large range of stresses, but for

cases of repeated loading of less than 20,000 cycles" (see AISC Commentary on Sec. 1.7) the values permitted are considerably below those based on the 1/3

increase in allowable stresses specified in Sec. 1.5.6. Th3 discrepancy

between Sec. 1.5.6 and Sec. 1.7.2 allowable stresses is illustrated in Table

E.l .

Table E.l ALLOWABLE STRESSES IN FILLET AND PARTIAL

PENETRATION TRANSVERSE WELDS

Stresses in Ksi

Servi ce Load

(Monotoni c)

Sec. 1.5.3

1 . 33 X Servi ce

Load

Sec. 1.5.6

Low-Cycle Fatigue

Sec. 1 .7.2

R = 0* R = -1*

18 24 22.5^1)

25.0(2)

11.25(1)

12.5

21 28 22.5^1)

25.0^2)

11.25(1)

12.5 (2)

*R is the stress reversal ratio.

(1) Partial penetration groove welds.

(2) Fillet Welds.

In the light of this inconsistency and because of possible distress due

to low cycle fatigue in elements subjected to stress reversals into inelastic

range, and also taking into account uncertainties regarding the intensities

of extreme loading conditions, the 33% increase in allowable values seems unconservati ve for the local critical sections were full stress reversals

takes place under earthquake loading.

E . 2 Beam-Column Behavior

In the usual structural analysis and design calculations, the deflections

are assumed to be so small that they have a negligible effect on forces and

moments in the various structural elements. Thus, the so-called "first-order

theory" is developed neglecting the deformations of the structure. In some cases, particularly, in slender members involving compression combined with

bending, the effects of deflection may become non-linear with load increase,

and must be taken into account. This is the basis for the so-called "secondorder theory". The effect of the center deflection A in a beam subjected to

end moments M and axial compression P can be seen from the expression for

center bending moment M:

M = M + PA

0

330
Tlie increase in bending moment at the center, and the corresponding increase

in moments along the beam-column, cause an increase in end rotation 6, so that a moment- rotat i on curve Mq vs 9 appears as shown in Fig. E.l. The

hysteresis loop observed in cyclic repeated loading is also shown in Fig. E.l.

The Importance of the so-called PA effect depends on the type of analysis

employed: if second-order theory is used and values of P, M and A are properly calculated, then the shape of the load-deformation curve (or hysteresis

loop) will be based on the basic properties of the section (or element) - developing full plastic moment capacity. However, when first-order theory

is used to define M = Hq, then an apparent reduction in capacity and in energy

dissipation is obtained.

The P-A effect may be important from the point of view of overall frame

stability and of design for appropriate maximum moment. However, it shall not

be treated here in detail as it is not a characteristic of the structural

element, but rather a question of proper evaluation of forces and moments in

a structural system (selection of 1st order or 2nd order analysis).

E . 3 Local Buckling

The principal effect of local buckling during cyclic loading (within

limited number of cycles examined) is to reduce stiffness; the reduction in

load is small. Tests on cantilever beams [92] have shown that after a small

number of reversals with large inelastic deformations the overall stiffness

of the beam may be reduced, primarily due to local buckling. For example a cantilever steel beam (W18 50), Fig. E.2, which initially develops a deflection of about 0.25 in. at the free end when subjected to a concentrated load

of 25 kips, after about 12 cycles of reversed loading developed a deflection

of about 0.5 in. under the same load of 25 kips. This reduction in stiffness

occurred gradually, during 7-8 cycles of loading in the range of ±50 to 70

kips. Even though reduction of stiffness is significant, the capacity of the

beam was not affected. Examination of hystersis loops for the tests reported

to date indicates some progressive variation in the stiffness of the members.

Also, tests have shown that at high inelastic strains even when no local

buckling is exhibited during the initial loading - after a small number of

inelastic reversals local buckling may develop. Such local buckling tends to

develop large secondary inelastic deformations and with sufficiently large

number of cycles it may lead to low-cycle fatigue fractures.

For elements subjected to reversal of plastic deformation width-thickness ratios for steel shapes at the critical regions for load reversal may

have to be chosen more conservatively than for static loading. Damage

observed in the Cordova Building during the Alaska earthquake in 1964 supported these findings.

Therefore, a 25% reduction in wi dth - thi cknes s ratio specified in AISC

Sec. 2.7 is recommended under conditions of full reversal of plastic deformations. This leads to the following:

Table E.2 REDUCED WIDTH-THICKNESS RATIOS (b./t.) - CYCLIC

REVERSED LOADING ^ ^

Yield Strength Sec. 2.7 Proposed

Plastic desi gn- Earthquake designFy, ksi mono tonic loading cyclic load reversal

36 17 1 3 50 14 10.5

60 12.6 9.5

331
Ideal Plastic echanlsn

"A

e

'toraent Rotation Curve for Beam Columns

(After Ref. 93)

H

Load vs Joint Rotation Hysteresis Hoop

(After Ref. 9)

Fig. E.l Beam-Column Effect

332
p
E . 4 Structural Steel Shapes and Connections

For tension-compression cycling, with relatively small plastic deformations (without strain hardening) and with "compact" shapes which do not

exhibit local buckling at a low number of reversed cycles the steel elements

develop stable hysteresis loops similar to those shown in Fig. B.4. For

bending, within the conditions imposed above (no strain hardening and no local buckling), the same stable and ductile behavior is observed.

A limited number of tests on structural steel connections under cyclic

reversed loadings have been reported [8, 10]. These include tests on allbolted connections, all-welded connections, and on flange-welded and webbolted connections, beam flanges framed into column f 1 ange--produci ng column

bending in the strong direction. A few tests were conducted with beams

framing into the column web.

Test results indicated that from the standpoint of cyclic behavior

(energy dissipation and endurance) the welded connections performed best.

The hysteresis curves remained stable, and even with cyclic recurrence of

local buckling and some reduction in sti ff ness--the capacity of the connection was maintained. Generally failure occurred by cracking the flange near

the weld after a number of cycles of buckling. Connections using a moment

flange plate did not differ appreciably from the connections with flanges

welded directly to the column.

While properly designed and welded connections have demonstrated

excellent performance under reversed cyclic loading, defects either in design

or in welding quality can significantly reduce the capacity of connections

subjected to cyclic reversed loads. To achieve consistent high performance

of welded connections requires proper design and execution (welding sequence,

etc.) of all welds and limitation of plate thicknesses in welded joints in

accordance with weldability of different steels. The potential problem of

earthquake-induced stress cracking in welded connections, particularly in

elements built-up using thick plates, requires further investigation.

Hysteresis loops for moment resistant beam-to-column connections made

with A325 bolts show characteristic effects of slippage. Fig. E.3. It can be seen that some slippage was observed in the first half-cycle, and that with

repeated excursions into inelastic range - slipping was occurring at a lower

load. Following the slipping stage in each cycle the bolts came into bearing

contact with the holes (punched 1/16 in. oversize), and the overall deformation at a maximum load was approximately the same as that in the first cycle.

However cyclic loading and recurring slipping resulted in decreasing

friction and reduced amount of energy dissipation.

Excessive distortion and local buckling in the panel region of a frame

joint. Fig. E.4, can substantially increase the rotations at the joint and

trans 1 ati onal displacements of joints [9, 14]. To prevent excessive shear

distortion, doubler plates or diagonal stiffeners may be required.

Until substantiated or proved unwarranted by appropriate tests, the

shear stress caused by cyclic inelastic reversals of (U+L+E) loads in the

panel region surrounded by stiffeners and flanges on all four edges shall not

exceed 0.3 F (a 25% reduction in the normally allowed shear stress).

F. Concrete-Steel Composite

F . 1 Encased Construction - Beams and Columns

Encased concrete-steel construction is defined as one in which the steel

334
Bolted Connections

Fig. E.3 Hysteresis Loop-Welded and Bolted Connections

335
(a) Lateral Loading

Local Kinks

(b) Panel Distortion

Tie,. E.^ Problems Introduced at Beam-to-Colunn

Connection.

336
element is totally surrounded by concrete at least 2 inches thick and in which

adequate steel mesh (or cage) surrounds the steel beam. In accordance with

current specifications, the- behavior of such encased beams is considered to be

fully composite, relying on the natural bond between steel and concrete. The

present AISC Code provides for this type of construction for beams but not for

columns. The design of such beams is based on allowable stress criteria, with

some special provisions for the case when the steel beam acts independently of

encasement

.

The ACI Code specifically excludes the encased steel flexural members

from its scope, stating that such designs are covered by AISC. On the other

hand, in the absence of AISC provisions for design of encased columns, the

ACI Code contains provisions for composite compression members. Actually, it

provides for composite action under combined compression and bending of

columns - using ultimate strength design criteria. Also some provisions for

the special case of concrete-filled steel tubes is included in the ACI Code.

The problem of composite action of beam-column subassembly, i.e. the

behavior of a joint in an encased construction system is not covered by either

code and so handicaps the designer in proper detailing of a joint. Furthermore, the inconsistency of AISC working stress design provisions for beams vs. the ACI ultimate strength design provision for columns further handicaps

practical use of this type of construction.

The advantages claimed for the encased steel construction are: (1) high

fi re- res i stance rating, as the frame requires no special fire proofing; (2)

efficient utilization of concrete in compression, where its use is particularly economical; (3) increased stiffness of the composite system; (4) con- finement of structural steel by surrounding concrete and reinforcement,

preventing local buckling which controls width-thickness ratios which can be

used; and (5) large reserve shear and flexure strength under cyclic load

re ve rs al s . Studies of encased steel construction under cyclic reversed loading are very few, and most of these have been carried out in Japan, where this type

of construction has been widely used. The types of some test specimens used

in these studies are shown in Fig, F.l.

Although the effectiveness and the economy of encased steel construction

vs. other alternative systems is yet to be evaluated, it can not be done in

the absence of consistent and reasonably complete design criteria.

F . 2 Composite Concrete-Steel Construction

Composite construction defines a structural system in which interaction

of a concrete slab with a steel beam is accomplished by means of mechanical

devices (shear connectors). Usually, concrete slab becomes the compression

flange of the composite beam, and the steel section resists primarily the

tensile stresses. This basic concept clearly provides little or no advantage

when full reversal of bending moments controls the design. In the absence of

reversal - some advantage may be gained from composite construction, provided

the shear transfer mechanism can survive the cyclic inelastic loading.

F . 3 Composite Concrete - Concrete Construction

This form of construction usually involves use of precast concrete

elements acting in an integral manner with cast- i n-pl ace concrete. With

appropriately designed reinforcement at the interface, this form of construction should perform as well as a fully cast- i n-p 1 ace system. The current ACI

Code (Chapter 17) covers the design requirements for concrete composite

flexural members. Further development of this type of construction, with

337
Fl?. F.l Concrete Encased Structural Steel

Subassemblies

.

338
proper attention to composite action of the f rame-s 1 ab-wal 1 systems is of

particular importance.

G. Masonry

G . 1 Reinforced Masonry

Un re i n forced masonry buildings have demonstrated poor performance in numerous earthquakes and in other exposures to severe loadings. However,

performance of reinforced masonry is considerably better. Reinforced masonry,

from a basic point of view, is only a special case of reinforced concrete: it

deals with certain size and shape of precast elements, joined together with

steel reinforcement and cast-i n-pl ace concrete infill framework and interface.

The principal questions regarding proper design of reinforced masonry

systems are: where and how much reinforcement must be provided, and what are the required cast-i n-pl ace concrete infill and interface joints. Some recent

tests indicated that under essentially monotonic loading reinforced concrete

masonry may have adequate strength and ductility. Its performance under

cyclic reversal of loading still requires further study.

H. Conclusions and Recommendations

H . 1 General Remarks

The review summarizes current knowledge regarding behavior of structural

elements under seismic and similar extreme conditions, suggests some improvements in design criteria and discusses briefly some innovations for better

performance of structural elements under repeated reversed loadings in the

inelastic range.

Emphasis was placed on factors to be considered in design and on the

evaluation of the relative importance of these factors.

The scope of the discussion was limited by space limitations. Therefore

in addition to references cited directly in the text, additional sources of

data are given in the bibliography.

H .2 Desi gn

Review of the commonly used basic structural design codes (AISC, ACI,

and SEAOC) reveals varying degrees of specific provisions for seismic or other extreme loadings. AISC sets very few guidelines for design under

such conditions. ACI included provisions for seismic design in a special

Appendix for the first time in 1971. The provisions of this Appendix are based largely on two sources - the Blume, Newmark, Corning [13] and the SEAOC

(1968 version). In addition, results of studies based on investigations of

damage in recent catastrophic earthquakes ( Skop je-1 963 , Anchorage-1 964 , and

Caracas-1 967) were used in formulating the provisions of the Appendix. A number of the recommendations in the ACI Appendix and the SEAOC recommended

requirements are similar, although some differences exist, particularly

between the ACI 1971 and SEAOC 1971 versions. In the absence of other

guidelines SEAOC has provided the leadership in developing special provisions

for seismic design, and their recommendations provided the basis for UBC

seismic design criteria. Unquestionably the ACI Appendix is a major step

forward in the recognition of the special problems associated with earthquakeresistant design of concrete structures. Use of these design criteria, particularly the 1971 SEAOC recommendations, should result in structures which can

339
withstand severe loadings without collapse.

Although there has been only limited damage (with few remarkable

exceptions) in recently designed major structures - one can not rely on this

as a measure of design adequacy, because large urban centers have not experienced extreme loading conditions. Some suggestions for design modifications

which would further improve performance of structures under extreme conditions

are outlined in the preceding sections and are summarized below.

Gaps in the knowledge of the extreme loadings and of the structural

response to some of these preclude general agreement on the appropriate

criteria for design which would minimize damage under these conditions.

Because some of the design recommendations are based on extrapolation of

"evidence" and on subjective conclusions (judgment), they are often unacceptable to large professional groups responsible for design specifications and

codes. The dilemma most often faced by such groups is the extent to which

additional cost of extra caution ("extra" - in a sense that it may not be

necessary) in design is acceptable in the light of risk of severe damage

(loss of life and property).

In the absence of adequate design criteria, the designer must rely

heavily on his understanding of structural behavior under the extreme loading conditions and must accept the responsibility for subjective decision

based on this special knowledge.

In making the design decisions, the individual designer must take full

cognizance of the following: dynamic nature of the extreme loading conditions, inelastic response of the structure, behavior of the structure with

some zones of limited damage (cracking, local buckling, crushing, permanent

distortion, etc.), means of providing sufficient ductility and energy dissipation (i.e. eliminating conditions which may result in brittle behavior of

an element), means of preventing imperfections in the structure (quality

control in construction), and "repai rabi 1 i ty " of certain types of damage.

Some of the steps the designer can take which would help him in arriving

at a judicious decision are the following:

(a) perform a reasonably realistic analysis of the structure, which

would indicate the range of load variation (maximum and minimum

loads) ,

(b) investigate local ductility of critical regions, taking into

account possibility of loading reversals if so indicated by the

analysis; distinction should be made as to full, partial, or no reversal conditions,

(c) avoid forms of construction and/or provide special reinforcement

to prevent brittle modes of failure,

(d) prescribe control tests on materials to be used in construction

which would ensure the desired material properties and quality

of workmansh i p

.

Reinforced Concrete

Shear . To prevent brittle shear failures the element must be designed for

the maximum probable shear force. In ductile moment resistant frame elements

this shear must not be less than that required for developing a "mechanism"

action. In addition where the load is repeated a number of cycles, the shear

capacity of concrete shall be reduced as follows:

(a) full reversal - concrete shear resistance is reduced to zero;

340
(b) no reversal - concrete shear resistance is reduced to 1/2 that

for static loading.

Local Buckling . Sufficient ties should be used to prevent local buckling of

each steel bar subjected to compression stress. Where large inelastic strains

are anticipated (in the strain-hardening range) or where full stress reversals in the inelastic range are anticipated, the spacing of the ties will be

of the order of 7D to 5D. Hoops and ties must be suitably hooked or prevented

from opening after outer cover has spalled. They also must be tied closely

to the longitudinal bars which are braced by them.

Crushing Strain . Concrete should be capable of maintaining a compressive

stress of not less than 0.80 x compressive strength (ft) at a strain of 1.5

e corresponds to stress ft. This requirement on the ability of concrete to

develop a descending branch of the stress-strain diagram is minimal for ductile behavior of reinforced concrete. Laboratory tests should be performed

to establish fulfillment of this requirement.

Rei nf orci ng Stee

1

. While generally ASTM A615 reinforcing steel may be

adequate to provide the necessary ductility - special requirements on yielding prior to strain hardening, and on strain hardening characteristics are desirable. It must be emphasized that "over-strength" of steel reinforcement

(due to higher yield or strain-hardening at strain below 0.003) may allow

development of moments in excess of calculated yield or plastic moments, and

thus increase shear loads on the element. This may change the failure mode

from ductile to brittle. Therefore, in specifying reinforcing steel, it is

advisable to set limits on both minimum and maximum yield strength values.

Structural Steel Elements :

Al 1 owabl e Stresses . Because of possible distress due to low-cycle

fatigue in elements subjected to stress reversals the allowable stresses

should be well below the yield values. In the light of the uncertainties

regarding the intensities of extreme loading conditions (including earthquakes), the 33% increase in allowable values seems unconse rvati ve for the

local critical section where full stress reversal takes place under earthquake loading. The present AISC Specifications are ambiguous on this point.

In Sec. 1.5.6 fatigue conditions (including low-cycle fatigue) are excluded

from the 33% increase in allowable value, but stresses produced by wind or seismic loading are allowed such an increase.

Until substantiated or proved unwarranted by appropriate tests, the 33%

increase in allowable values should be clearly disallowed in all cases where

full stress reversal may occur in the inelastic range.

Width-thickness Ratios . In critical regions where significant reversals

of stresses can occur, special provisions must be made to minimize local

buckling and low-cycle fatigue fractures. The width/thickness ratio used in

compact sections are adequate to prevent (or minimize) local buckling at

plastic hinge locations under monotonic loading. Further reductions in widththickness ratios are indicated in regions where full reversal of stress takes

place under extreme conditions.

Until substantiated or proved unwarranted by appropriate tests, a 25%

reduction in width-thickness ratios specified in Sec. 2.7 of AISC Specification is suggested where full stress reversal may occur in the inelastic range.

Welded Connections . Properly designed and welded connections have

demonstrated excellent performance under reversed cyclic loading. However,

defects either in design or in welding quality can significantly reduce the

capacity of connections subjected to cyclic reversed loads. To achieve

341
consistent high performance of welded connections requires proper design and

execution (welding sequence, etc.) of all welds and limitation of plate thicknesses in welded joints in accordance with weldability of different steels.

Non-destructive inspection of welds should be specified in critical

regions, particularly in connections with thick steel plates.

Panel Distortion . Excessive distortion and local buckling in the panel

region of a frame joint can substantially increase the rotations at the joint

and trans 1 ati onal displacements of joints. To prevent excessive shear distortion doubler plates or diagonal stiffeners may be required.

Until substantiated or proved unwarranted by appropriate tests the shear

stress caused by (D+L+E) loads in the joint panel surrounded by stiffeners and

flanges on all four edges shall not exceed 0.3 F (a 25% reduction in the

normally allowed shear stress).

P-A Effect . Frame deformation and joint displacements must be considered

in evaluating the forces and moments at critical section. This can be done

using "second-order" theory for analysis, or using approximate methods (e.g.

an iterative procedure, or amplification factors).

H . 3 Research

Considerable research has been carried out on behavior of structural

elements under repeated and reversed loadings of high intensity. These

investigations greatly increased the understanding of the problems and

provided a basis for some improvements in design. However, the existing

information has three major limitations:

(1) significant gaps exist in our knowledge of behavior of certain

types of elements ,

(2) laboratory studies have been largely limited to tests on isolated

elements, rather than on full scale structures or on full scale

subassembl i es ,

(3) loading histories used in the studies reported to date have not

been adequately related to the actual actions (load histories)

experienced by the structural systems under real earthquakes or other extreme conditions.

Therefore among the highest priorities in additional research would be

studies on more realistic subassemblies (modeling portions of full scale

structures). For example, the nature and magnitude of the contribution made

by the floor slab to the behavior of the frame (beam and column) subassembly

are essentially unknown. Further studies of this interaction may lead to more economical design.

Even more important, behavior of real buildings as systems subjected to

realistic extreme loadings must be studied. For this purpose a special program must be developed, which would provide for coordinated analytical,

laboratory and field studies.

Choice of loadings is equally important. Fig. H.l illustrates some typical loadings used in studies of some steel connections. The magnitudes

of peak loads and the sequence of cycles was selected somewhat arbitrarily.

While no one set could be prescribed as a standard, guidelines should be

developed for selecting loading conditions representing idealizations of real

structures' response.

Furthermore, a number of special problems require further study. It is

342
difficult to list these in a precise order of priority. Some of the high

priority problems are indicated below.

Ductility and Limited Damage . The concept of ductility, while useful

in some aspects of design, does not adequately reflect the dynamic (timedependent) nature of special loading conditions and it does not properly

reflect energy dissipation characteristics. It may also be misleading as in some cases it requires extensive local damage to develop reasonable overall

ducti 1 i ty

.

Prescribed ductility factors should be reviewed, their definition

clarified, and particularly their relationship to local damage closely

exami ned

.

Guidelines for Evaluation of Safety of Existing Buildings . Many existing buildings while not entirely conforming to new codes reflecting current

structural forms and practices, may nevertheless be safe - as a consequence

of contributions from non-structural portions of the system. Guidelines for

evaluation of their safety would be useful to communities wishing to minimize

overall hazards to people and structures during extreme loadings.

Performance of Melded Thick Steel Plate Elements . Some concern has been

expressed for potential vulnerability of such structural elements to lowcycle fatigue. Insofar as these elements are now widely used in high rise

structures - evaluation of the performance of such elements is particularly

important

.

Residual Safety of Structures . Major earthquakes are likely to damage

many structures . In view of the uncertainty of repair effectiveness - safety

of residual (repaired or unrepaired) structures is particularly important.

For example, changes in the local rigidity (stiffness) of joints may have

decisive influence on stability of the structure during subsequent extreme,

or even moderate, lateral loadings.

In addition many details of design, material behavior, imperfections

inherent in fabrication and construction, and other characteristics of

specific structural elements and systems require further study. Some of

these have been identified in preceding sections; a complete list is entirely

beyond the scope of this survey.

Bi b1 i ography

Newmark, N. M., and Rosenblueth, E., " Fundamentals of Earthquake

Engi nee ri ng , " Prentice-Hall, Engl ewood Cliffs, N . J . , 1 971 , 640 p

.

Sargin, M. , "Stress-Strain Relationships for Concrete and the Analysis

of Structural Concrete Sections," University of Waterloo, Waterloo,

Ontario, Canada, Solid Mechanics Division, Study , No. 4 , 1 971 , 1 67 p. Cohn , M. Z., and Ghosh, S. K. , "The Flexural Ductility of Reinforced

Concrete Sections," University of Waterloo, Ontario, Canada, Solid

Mechanics Division, Report , No. 100, November 1971 , 24 p.

ACI Committee 318, "ACI Standard Building Code Requirements for

Reinforced Concrete (ACI 318-71) American Concrete Institute, Detroit,

Michigan, 1970.

ACI Committee 318, "Commentary on Building Code Requirements for

Reinforced Concrete (ACI 318-71) American Concrete Institute, Detroit,

Michigan, 1970.

343
Fig. M.l Loadinc Histories for Selected Tests

344
6. Joint Committee of the Welding Research Council and the American Society

of Civil Engineers, " Plastic Design in Steel, A Guide and Commentary ," 2nd edition, ASCE, New York, 1971, 336 p.

7. Popov, E. P., "Low-Cycle Fatigue of Steel Beam-to-Col umn Connections,"

RILEH, International Symposium on the Effects of Repeated Loading of

Materials and Structural Elements, Mexico City, September 15-17, 1966,

13 p.

8. Popov, E. P., and Stephen, R. M., "Cyclic Loading of Full-Size Steel

Connections," American Iron and Steel Institute, Bui 1 et i n , No. 21,

February 1972, 37 p.

9. Krawinkler, H., Bertero, V. V., and Popov, E. P., "Inelastic Behavior

of Steel Beam-to- Col umn Subassembl ages , " University of California,

Berkeley, Earthquake Engineering Research Center, Report , No. EERC

71-7 , October 1 971 , 290 p

.

10. Popov, E. P., and Pinkney, R. B., "Behavior of Steel Building Connections

Subjected to Repeated Inelastic Strain Reversal - Experi mental Data,"

University of California, Berkeley, Department of Civil Engineering,

Structures and Materials Research, Report , No. SESM 67-31, December 1 96 7 , 324 p.

11. Popov, E. P., and Pinkney, R. B., "Behavior of Steel Building Connections

Subjected to Repeated Inelastic Strain Reversal," University of

California, Berkeley, Department of Civil Engineering, Structures and

Materials Research, Report, No. SESM 67-30, December 1967, 74 p.

12. Hisada, T., Ohmori , N., and Bessho, S., "Earthquake Design Considerations

in Reinforced Concrete Columns," Kajima Institute of Constructi o-n Technology, Tokyo, KICT Report , No. 1, January 1972, 20 p.

13. Blume, J. A., Newmark, N. M., and Corning, L. H., " Design of Mult i -Story

Reinforced Concrete Buildings for Earthquake Motions ," Portland Cement

Association, Chicago, 1961, 318 p.

14. Bertero, V. V., Popov, E. P., and Krawinkler, H., "Beam-Column Subassemblages Under Repeated Loading," ASCE Proc . Vol. 98, No. ST 5,

May 1972, pp. 1137-1159.

15. Climenhaga, J. J., and Johnson, R. P., "Moment-Rotation Curves for

Locally Buckling Beams," ASCE Proc . Vol. 98, ST 6, June 1972, pp.

1239-1254.

16. Popov, E. P., and Pinkney, R. B., "Cyclic Yield Reversal in Steel

Building Connections," ASCE Proc . Vol. 95, ST 3, March 1 969 , pp. 327-

353.

17. Hanson, N. W., "Seismic Resistance of Concrete Frames with Grade 60

Reinforcement," ASCE Proc. Vol. 97, ST 6, June 1971, pp. 1685-1700.

18. Aoyama, H., "Moment-Curvature Characteristics of Reinforced Concrete

Members Subjected to Axial Load and Reversal of Bending," with

Discussion by K. H. Gerstle aiid L. G. Tulin and H. Aoyama, International Symposium on Flexural Mechanics of Reinforced Concrete,

ASCE-ACI, Miami, Florida, November 1 0-1 2 , 1 964 , Proceedi ngs , pp. 183-

212. (ASCE 1 965-50 ; ACI SP-12) .

19. Roy, H. E. H., and Sozen , M. A., "Ductility of Concrete," International

Symposium on Flexural Mechanics of Reinforced Concrete, ASCE-ACI, Miami,

Florida, November 1 0-1 2 , 1 964 , Proceedings , pp. 21 3-224. (ASCE 1 965-50;

ACI SP-12) . 345
20. Medearis, K. , and Young, D. H., "Energy Absorption of Structures Under

Cyclic Loading," ASCE Proceedings , Vol. 90, ST 1, February 1 964 , Part 1,

pp. 61-91 .

21. Yamada, M., "Verhalten Plastischer Gelenke in Stahl betonsaeul en ,

(Behavior of Plastic Hinges in Reinforced Concrete Columns) lABSE

Seventh Congress, Rio de Janeiro, Brazil, 10-16 August 1964, Final

Report , lABSE General Secretariat, Zurich, 1965, pp. 435-442.

22. Popov, E. P., and Franklin, H. A., "Steel Beam-to- Col umn Connections

Subjected to Cyclically Reversed Loading," Preliminary Reports Presented

at the 1965 Annual Meeting of the Structural Engineers' Association of

California, American Iron and Steel Institute, New York, February 1966,

pp. 6-9.

23. Beedle, L. S., "Reversed Loading of Steel Frames--Prel iminary Tests,"

Preliminary Reports Presented at the 1965 Annual Meeting of the

Structural Engineers' Association of California, American Iron and

Steel Institute, New York, February 1966, pp. 1-5.

24. Brown, R. H., and Jirsa, J. 0., "Reinforced Concrete Beam Under Load

Reversals," ACI Journal, Proc . Vol. 68, No. 39, May 1971, pp. 380-390.

25. Hanson, N. W., and Conner, H. W., "Seismic Resistance of Reinforced

Concrete Beam-Column Joints," ASCE Proc . Vol. 93, ST 5, October 1967,

pp. 533-560.

26. Bresler, B., and Bertero, V. V., "Behavior of Reinforced Concrete Under

Repeated Load," ASCE Proc . Vol. 94, ST 6, June 1968, pp. 1567-1590.

27. Sinha, B. P., Gerstle, K. H., and Tulin, L. G., "Stress-Strain

Relations for Concrete Under Cyclic Loading," ACI Journal, P roc . Vol.

61, No. 12, February 1964, pp. 195-211.

28. Sinha, B. P., Gerstle, K. H., and Tulin, L. G., "Response of Singly

Reinforced Beams to Cyclic Loading," ACI Journal, Proceedings Vol. 61,

No. 56, August 1964, pp. 1021-1038.

29. Agrawal , G. L., Tulin, L. G., and Gerstle, K. H., "Response of Doubly

Reinforced Beams to Cyclic Loading," ACI Journal, Proceedings Vol. 62,

No. 51, July 1965, pp. 823-835.

30. Bertero, V., "Inelastic Behavior of Beam-To-Col umn Subas sembl ages Under

Repeated Loading," University of California, Berkeley, Earthquake

Engineering Research Center, Repo rt , No. EERC 68-2, April 1 968 , 65 p.

31. Bertero, V. V., Bresler, B., and Liao, H., "Stiffness Degradation of

Reinforced Concrete Members Subjected to Cyclic Flexural Moments,"

University of California, Berkeley, Earthquake Engineering Research

Center, Report , No. EERC 69-12, December 1969 , 1 1 5 p.

32. Ohno, K., and Shibata, T., "A Consideration on the Damages to Columns

of Reinforced Concrete by the Tokachioki Earthquake, 1968," Proceedings

of the U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the

Safety of School Buildings, 21-26 September 1970, Sendai, Japan; Japan

Earthquake Engineering Promotion Society, Tokyo, 1971, pp. 152-155.

33. Aoyama, H. et al . , "A Study on the Cause of Damage to the Hachinohe

Technical College Due to 1968 Tokachi-Oki Earthquake (Part 1)," Proc . of the U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the Safety of School Buildings, 21-26 September 1970, Sendai, Japan;

Japan Earthquake Engineering Promotion Society, Tokyo, 1971, pp. 199-

212.

346
34. Higashi , Y., and Ohkubo, M., "Static and Dynamic Loading Tests of

Reinforced Concrete Frames with Thin Spandrel or Wing Walls," Proc . of the U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the Safety of School Buildings, 21-26 September 1970, Sendai , Japan;

Japan Earthquake Engineering Promotion Society, Tokyo, 1971, pp. 225-

239.

35. Ohsaki , Y., Watabe , M., and Matsushima, Y., "Experimental Study on Five-Story Full Size Apartment House of Reinforced Concrete Walled

Frames," Proceedi ngs of the U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the Safety of School Buildings, 21-25 September

1970, Sendai, Japan; Japan Earthquake Engineering Promotion Society,

Tokyo, 1971 , pp. 240-266.

36. Shiga, T., et al . , "The Dynamic Properties of Reinforced Concrete

Frames," Proceedi ngs of the U.S. -Japan Seminar on Earthquake

Engineering with Emphas i s on the Safety of School Buildings, 21-26

September 1970, Sendai, Japan; Japan Earthquake Engineering Promotion

Society, Tokyo, 1971 pp. 346-363.

37. Wakabayashi , M., "The Behavior of Steel Frames with Diagonal Bracings

Under Repeated Loading," Proceedi ngs of the U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the Safety of School Buildings,

21-26 September 1970, Sendai, Japan; Japan Earthquake Engineering

Promotion Society, Tokyo, 1971, pp. 328-345.

38. Kokusho, S., and Ogura, K., "Shear Strength and Load Deflection

Characteristics of Reinforced Concrete Members," Proceedi ngs of the

U.S. -Japan Seminar on Earthquake Engineering with Emphasis on the

Safety of School Buildings, 21-26 September 1970, Sendai, Japan; Japan

Earthquake Engineering Promotion Society, Tokyo, 1971, pp. 364-381.

39. Bresler, B., "Lightweight Aggregate Reinforced Concrete Columns,"

ACI Paper SP 29-7, 1971, pp. 81-130.

40. Gerstle, K. H., and Tulin, L. G., "Shakedown of Continuous Concrete

Beams," RILEM, International Symposium on the Effects of Repeated

Loading of Materials and Structural Elements, Mexico City, September 15-

17, 1966.

41. Alatorre, G., and Casillas, J., "Shear Strength Behavior of Concrete

Beams Subjected to Alternate Loads," RILEM, International Symposium on the Effects of Repeated Loading of Materials and Structural Elements,

Mexico City, September 15-17, 1966, 10 p.

42. Muguruma, H., Tominaga, M., and Okamoto, S., "Mechanical Properties of

Simply Supported Prestressed Concrete Beams Under Repeated Loadings,"

RILEM, International Symposium on the Effects of Repeated Loading of

Materials and Structural Elements, Mexico City, September 15-17, 1966,

9 p.

43. Crist, R. A., "Shear Behavior of Deep, Reinforced Concrete Beams,"

RILEM, International Symposium on the Effects of Repeated Loadings of

Materials and Structural Elements, Mexico City, September 15-17, 1966,

31 p.

44. Esteva, L., "Behavior Under Alternating Loads of Masonry Diaphragms

Framed by Reinforced Concrete Members," RILEM, International Symposium

on the Effects of Repeated Loading of Materials and Structural Elements,

Mexico City, September 15-17, 1966, 21 p.

45. Arnold, P., Adams, P. F., and Lu , L., "The Effect of Instability on the

Cyclic Behavior of a Frame," RILEM, International Symposium on the

Effects of Repeated Loading of Materials and Structural Elements,

347
Mexico City, September 15-17, 1966, 24 p.

46. Medearis, K. , "Static and Dynamic Properties of Shear Structures,"

RILEM, International Symposium on the Effects of Repeated Loading of

Materials and Structural Elements, Mexico City, September 15-17, 1966,

19 p.

47. Borges , J. F., and Ravara, A., "Structural Behavior of Panel Structures

Under Earthquake Actions," RILEM, International Symposium on the Effects

of Repeated Loading of Materials and Structural Elements, Mexico City,

September 15-17, 1966, 9 p.

48. Yamada, M., and Nakaza, I., "Low Cycle Fatigue of Riveted and High

Strength Bolted Joints in Steel Beams," RILEM, International Symposium

on the Effects of Repeated Loading of Materials and Structural Elements,

Mexico City, September 15-17, 1966, 14 p.

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348
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71. Yamada, M. , "Low Cycle Fatigue Fracture Limits of Various Kinds of

Structural Members Subjected to Alternately Repeated Plastic Bending

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72. Kartsivadze, G. N., and Avalishvili, L. N., "Research on Behavior of

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73. Shiga, T., and Ogawa, J., "The Experimental Study on the Dynamic

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74. Wakabayashi , M. , Nonaka, T., and Matsui , C, "An Experimental Study on the Horizontal Restoring Forces in Steel Frames Under Large Vertical

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76. Bresler, B., and Gilbert, P. H., "Tie Requirements for Reinforced

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78. Park, R., Kent, D. C, and Sampson, R. A., "Reinforced Concrete Members

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Technical Committee 21, 21 p.

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81. Burns, N. H., and Siess, C. P., "Repeated and Reversed Loading in Reinforced Concrete," ASCE Proceedi ngs , Vol. 92, No. ST 5, October 1 966 , pp. 65-78.

82. Clough, R. W., "Effect of Stiffness Degradation on Earthquake Ductility

Requirements," University of California, Berkeley, Structural Engineering Laboratory, Repo rt No. 66-16, October 1966 , 67 p.

83. Richart, F. E., Brandtzaeg, A., and Brown, R. L., "A Study of the

Failure of Concrete Under Combined Compressive Stresses," University

of Illinois Engineering Experimental Station, Bulletin , No. 185, 1928,

104 p.

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by Roy, H. E. H., and Sozen , M. A., International Symposium on Flexural

Mechanics of Reinforced Concrete, ASCE-ACT, Miami, Florida, November 10-

12, 1964, Proceedings , pp. 227-234. (ASCE 1965-50; ACI SP-12).

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Concrete Members," University of Illinois, Engineering Experiment

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and Spirally Reinforced Concrete in Compression," University of Illinois,

Engineering Experiment Station, Bulletin , No. 190, April 1 929 , 72 p.

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Concrete Columns," University of Illinois, Engineering Experiment

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90. Bertero, V. V., "Experimental Studies Concerning Reinforced, Prestressed

and Partially Prestressed Concrete Structures and Their Elements,"

Introductory Report for Theme IV of the lABSE Symposium on Resistance

and Ultimate Def ormab i 1 i ty of Structures Acted on by Repeated Forces,

Lisbon, September 1973. (In Press)

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ASCE Proceedings , Vol. 94, No. ST 1, January 1 968, pp. 267-381.

92. Bertero, V. V., and Popov, E. P., "Effect of Large Alternating Strains

on Steel Beams," ASCE Proceedi ngs , Vol. 91, ST 1, F-ebruary 1 965 , pp. 1-

12 .

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Structures in the Inelastic Range," AISI Bulletin No. 8, March 1968,

42 p.

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and the National Bureau of Standards, August 28-September 1,

1972, Boulder, Colorado (Issued February 1973).

351
BEHAVIOR OF STRUCTURAL SYSTEMS UNDER DYNAMIC LOADS

by

Roland L. Sharpe', Garrison Kost^ , and James Lord^

1. Introduction

Structural systems may be subjected to many different types of dynamic

loads. These may include wind, blast, and impact loading, earthquake ground

motion, marine wave action, sonic boom pressures , and mach i nery- i nduced

vibrations. Of these loadings, only the earthquake ground motion can affect,

nearly simultaneously, all components of a structure including its contents.

The techniques and procedures which have been developed for analysing earthquake-induced motions in structures can also be utilized in determining

structural response to other types of dynamic loading. Therefore, this paper

is devoted to presenting a comprehensive review of current and state-of-theart procedures used in seismic analysis and design of various types of

structural systems. Available research results are evaluated in light of

current practice and areas needing improvement or research are discussed.

The advent of the computer age has made—it possible to better analyse

and design structures subjected to dynamic loading. Detailed mathematical

modeling techniques have been developed so that almost any structural con- figuration can be modeled to realistically represent its physical characteristics. The model can then be subjected to various types of dynamic loads to

compute its response. Once the response has been obtained, the individual

members or components can be checked to ensure their adequacy to resist the

imposed loads. If certain members are inadequate, they can be modified, and

the analysis and checking repeated. Thus the complete design process for

earthquake loadings consists of analysis-design cycles as do the processes

for other more conventional loadings.

Considerable attention has been given to dynamic analysis with respect

to the design of high rise structures; but the structures in which perhaps

90 percent of the people live, work, or sleep - the low rise and often non- engineered structures - have largely been ignored. With this in mind, the

considerations affecting the behavior of this type of structures are discussed

in this paper in addition to the more exotic or glamorous high rise buildings.

This paper presents first a general review of analytical methods,

followed by a discussion of structural configurations and modeling procedures.

Applications of the analytical methods are then presented, and many of the

factors to be considered in the dynamic analysis and design of a structure

are discussed. As equipment and systems supported in structures are often

important to public safety, equipment-structure interaction and suggested

analytical techniques are also discussed. Finally, present philosophy in

practice and trends in design, together with possible simplified design

techniques, are reviewed.

Executive Vice President, Operations, John A. Blume & Associates, Engineers.

Assistant Vice President, John A. Blume & Associates, Engineers.

Director of Systems Engineering, A. C. Martin & Associates.

352
II. General Review of Seismic Analysis Methods

Earthquakes are a global phenomenon as seismic activity is not confined

to any one country or even one continent. Until about 50 years ago, earthquakes were largely considered to be acts of God, and no specific provisions

were written into any code of practice to protect against earthquake hazards.

Since that time, many methods of analyzing structural systems for seismic

loads have been devised and are presently in use by engineers around the

world. Most earthquake-prone countries have adopted building codes requiring

some form of seismic analysis to guard against earthquake losses associated

with the ground shaking hazards. These codes generally accept approximate

equivalent static analysis methods.

The introduction of the high-speed digital computer in the sixties

spurred the development of many computer programs which are capable of predicting the dynamic behavior of linearly elastic structural systems subjected

to a prescribed earthquake ground motion. Subsequent program development has

taken place that enables engineers to predict the inelastic behavior of

idealized structures when vibrating under the action of predetermined seismic

loads. Presently, new methods and techniques are being developed which assume earthquakes as being random events and as such, probabilistic predictions of

dynamic behavior can be made. To date, however, most transfer functions used

in making predictions of this nature have been assumed deterministic and

linear in themselves. Before reviewing methods of dynamic analysis, the

methods used to determine equivalent static seismic forces will be discussed.

A . Design Seismic Forces

The behavior of a structure when vibrating under the influence of seismic motions is dynamic in nature. Without the aid of a computer it is

impractical, if not impossible, to determine the detailed dynamic behavior

of structures under such conditions. Thus, in the past, engineers, while

recognizing the dynamic nature of the problem, have sought to simplify and

reduce the problem to a static one; dynamic forces are represented by codeprescribed equivalent static horizontal forces. The criteria are based

largely on uniform type structures with symmetrical loadings. Present codeprescribed design seismic forces are not the seismic forces that can be

expected in a structure during a major earthquake. Factors such as structural

damping, ductility, participation of "non-seismic" elements, and the customary

safety factors used in design are relied upon to ensure survival of structures

in seismic events. The objectives of such designs are:

0 to prevent structural damage and minimize nonstructural damage in

small to moderate earthquakes

• to prevent structural collapse during great earthquakes

0 to generally reduce hazards to public safety.

The general procedures used in the determination of code seismic lateral

forces and the distribution of such forces throughout the structure are of

interest. Static seismic design forces generally are equal to a certain

percentage of the total weight of the structure and are applied, non-currently, along each of the principal horizontal axes of the structure. Many

factors have been introduced that influence the magnitude of such lateral

forces. These factors include dead plus other real loads, seismic zone factors, building type factors, fundamental periods of vibration of the

structure, site soil factors, structure importance or occupancy factors, and

foundation type factors.

Two methods are in general use for distributing the seismic lateral

forces over the height of a structure. The first method requires that the

353
total lateral force be computed for the structure using an appropriate base

shear coefficient. This total force is then distributed, vertically, such

that the sum of the static lateral forces applied at each level equals the

total lateral force or base shear on the entire structure. The pattern of

this distribution is usually governed by the overall dimensions of the

structure or the nature, size, and location of any prevailing setback con- ditions.

The second method requires that the individual story lateral forces be

directly determined using a prescribed lateral seismic coefficient for the

particular level or story concerned. Normally, such lateral coefficients

vary over the height of the structure in an attempt to account for the

dynamic behavior of the structure during an earthquake. In this case, the

total base shear is the summation of the individual story forces.

Once the individual story lateral forces are determined, the distribution

of such forces to the various elements in the lateral force resisting system

is usually accomplished by computing individual element forces in proportion

to their relative rigidities. In some analyses, the rigidity of the horizontal bracing system or diaphragm is considered in the distribution. This

implies that in the case of very flexible horizontal diaphragms, the distribution is done essentially on a tributary area basis. There are a number of

approximate method for determining such distributions as discussed in the

following paragraphs.

B . Approximate Static Analyses

Once the code forces are determined, it is necessary to perform analyses

of the lateral force resisting elements, frames or shear walls, to obtain the

forces to be resisted by each so that their adequacy can be ascertained. Many

approximate methods have been devised and implemented for distributing the

lateral forces. These will be discussed briefly.

For rigid frame structures, there are a number of approximate analysis

methods, and these may be classified into three groups:

Group 1 : Portal Method

Cantilever Method

Group 2 : Spurr Method

Modified Portal Method

Group 3 : Joint Coefficient Method

Factor Method

Group 1

The Portal Method and Cantilever Method incorporate basic approximations

which disregard frame dimensions and sizes. They assume that points of con- traflexure exist at the mid-points of both girder and column elements when

subjected to lateral loads alone.

The portal method further assumes that all interior columns carry the same shear load and that exterior columns carry only half the shear of an interior column. Thus, the portal method implies that column rotations at

the joints above and below are the same as that of the floor level under

consideration, stiffnesses of all columns are the same, stiffnesses of all

girders are the same, columns are very rigid in comparison to the girders,

and vertical supports are unyielding.

The cantilever method does not assume shear distribution directly. It assumes that column vertical loads due to overturning moments vary as the

354
distances of the columns from the centerline of the frame. Horizontal shear

distribution in columns is then computed from the resulting vertical shears

in the girders. Inherent in this method is the assumption that the girders

are infinitely rigid.

The rationale for using the portal rather than the cantilever method

depends greatly on the nature of the frame being analyzed. Most frequently,

these methods are used only in preliminary sizing of elements.

Group 2

The second group of methods involve modifications to the portal and

cantilever methods. Modifications include partial provisions for frame

dimensions, element sizes, and deflections.

The Spurr Method modifies the Cantilever flethod to size elements so as to improve the accuracy of the assumptions of the cantilever method. First,

vertical shears in girders are adjusted to reflect actual column areas. The

girders are then sized to produce equal girder drift contributions under the

established shears. The method features a basic advance from Group 1 methods

by considering drift and shears together. However, it represents a backward

approach to the column-girder compatibility because it assumes the column

vertical deformations to be of primary importance.

The Modified Portal Method presents an improvement in that points of

contraf Texture are not necessarily assumed at the mid-height of the columns

located in the top and bottom stories, but usually at 0.6 of the story height.

Group 3

The third group represents a higher order of shear distribution methods

for use in slide rule or desk calculator computations. Approximate provisions

for individual element sizes, frame dimensions and deflections are included.

The Joint Coefficient Method assumes points of contraflexure at the

midpoints of all girders and columns, and shear is distributed in proportion

to joint coefficients computed for each joint. This method is perhaps the

one most commonly used in slide rule computations. Care has to be exercised

in using this method for irregular frames, especially at points of discontinuity.

The Factor Method is a refinement of the joint coefficient method. A partial carry-over system is used to provide for variations in the location

of points of contraflexure.

For shear wall or box type structures the relative rigidities of each

of the piers and wall elements comprising the lateral force resisting system

are usually computed. Such rigidities generally include the summation of

components associated with both shear and flexural deformations of individual

members and the wall as a whole. Wall openings are, of course, always to be

accounted for.

C . Detailed Static Analyses

The previously discussed methods of static analysis for code seismic

analyses are suitable for sliderule calculations or desk-top calculator

procedures. Recent trends indicate that more accurate computer solutions

using matrix methods are being used to distribute horizontal shears between

lateral force resisting elements and to determine structural member internal

forces. This may reflect, in many cases, recent more stringent code requirements that require interaction between shear wall and frame elements to be

355
considered. Finite element techniques utilizing truss, beam-column and plate

elements are sometimes used.

The basic analytical matrix procedure used for these static analyses is

the displacement (or stiffness) method. This method is used for linearly

elastic structures, and is well documented in the literature [48, 54, 70, 75].

The basic assumptions of the displacement method are that the materials are linearly elastic (i.e., obey Hooke's law), the displacements of the structure

are small (all computations can be made on the basis of the original geom - try), and that the effects of axi al -f lexural interactions can be neglected.

Using these assumptions, the equations of joint equilibrium (or superposition

equations) for a static analysis can be solved by any of a number of procedures (such as decomposition) to obtain the unknown joint displacements.

Once the joint displacements are obtained, the member internal forces can be

calculated by conventional means.

D . Deterministic Dynamic Analysis

An analysis is deterministic when both the configuration of a structure

and the applied loads can be uniquely described. It is now generally accepted

that mathematical models can be devised to reasonably represent real structures when subjected to strong earthquake ground motions. An analysis is

considered to be dynamic if it recognizes that both loading and response are time dependent and employs a suitable method capable of simulating and

monitoring such time dependent behavior. The following is a brief review

of the various aspects involved in a dynamic analysis.

The dynamic behavior of any structural system is governed not only by

the nature of the applied loading but also by the dynamic characteristics of

the structural system itself. The dynamic characteristics of a linearly

elastic structure are commonly represented by the natural frequencies and

mode shapes of the structure. For every degree of freedom in a linearly

elastic structure, there exists a unique natural frequency of vibration and

an associated mode shape. It is necessary to obtain these dynamic characteristics if the modal superposition method is used to compute the dynamic

response .

The fundamental frequency (or period) of the structure may be estimated

by the use of code-prescribed approximate formulae, where the period is

usually expressed as a function of building dimensions. When more accuracy

is required, then computer programs which provide for the automated calculation of natural frequencies and mode shapes must be used. The calculation of

such values by hand is usually only practical for very small problems.

Because earthquake ground motions are erratic in nature, they cannot be

represented mathematically by a single continuous function for which a closed

form solution can be obtained. They can be described in digital form. Most

analytical methods that can utilize digitized load functions fall into two

broad categories; Ilodal Superposition Methods or Step-by-Step Integration

Methods.

The r^odal Superposition Method recognizes that for linearly elastic

systems vibratingwithin the elastic range, there exists a unique characteristic mode of vibration corresponding to each degree of freedom in the

system. Furthermore, because the principle of superposition holds, the total

response of the system can be represented by the sum of the individual

responses from each of the characteristic modes. The method allows the

equations of motion to be "uncoupled" and treated as a number of independent

equations, each of which is analogous to the equation of motion for a single

degree of freedom system. The modal equations of motion are then solved for

each of these independent systems. Each of the modal equations are integrated

to determine the response parameters for the modes under consideration at each

356
time step, and the response parameters for the physical structure at each time

step can be obtained by transforming the modal quantities to physical coordinates. Once the displacements of the structure are calculated, the remaining

response parameters such as element forces or stresses can be computed by

conventional means. The modal superposition, time history method is well

accepted, efficient, and widely used by many engineers.

As an alternative, the response spectrum method can be used to solve the

equations of motion. This method provides an estimate of the maximum re- sponse of a structure and does not require the computation of the complete

time-history of the response. A response spectrum is a plot of the maximum

values of some parameter, such as acceleration, experienced by a family of

si ngl e-degree-of -freedom systems, when the systems are subjected to a timehistory of input ground motion; the maximum values are expressed as a function

of the natural period or frequency and damping of the systems. The engineering significance of the response spectrum lies in the fact that once the

spectrum for an earthquake is known, it is possible to compute the forces

induced in a si ngle-degree-of freedom system by that earthquake. It is

possible to extend the usefulness of the response spectrum to mul ti -degreeof-freedom systems by utilizing the modal superposition method of dynamic

analysis.

Spectra are usually generated for single-degree-of-f reedom (SDOF) linear

elastic systems. They are occasionally developed for mul ti -degree-of-f reedom

elastic systems and non-linear systems. In addition they can be calculated

with time as a third dimension to monitor variations in response maxima with

ti me

.

For simple structures such as elevated tanks or single story structures,

SDOF response spectra can be used directly to obtain the maximum structural

response. For more complicated mul ti -degree-of-f reedom structures the maximum response in each mode is computed. The maximum values of the various

response parameters must then be estimated because the time at which the

maximum response occurs is not preserved in the spectral curves. Since the

maximum values of individual modes do not necessarily occur at the same time nor have the same sense, the exact way in which various modes combine cannot

be precisely determined. Commonly suggested methods of modal combination are

to compute the sum of the absolute values of all significant modes or to

compute the square root of the sum of the squares of each individual modal

response. Other methods that have been used include the computation of the

square root of the sum of the squares of the largest individual modal contribution plus one half of the sum of the squares of all other contributions or the sum of the absolute values of two or more of the significant modes. The

first method yields an upper bound. The other methods should be used with

care because they can yield unconservati ve results in certain instances.

The Step-by-Step Integration Methods involve the direct numerical

integration of the equations of motion. These methods incorporate general

techniques and as such are applicable to elastic or inelastic systems. It

is not necessary to solve for frequencies or mode shapes nor is it necessary

to transform the system of equations to normal coordinates. Changes in

geometric, material, or connectivity properties of the structure may be made

at any time step by modifying the appropriate mass, damping, or stiffness

paramete rs

.

The cost of implementing an analysis of this type relates directly to

the size of the time step used in the integration. In general, the larger

the time step the greater the likelihood of obtaining an unstable or inaccurate solution. Thus, a balance between cost and accuracy is frequently a principal concern, especially in the case of large structures.

The development of numerical integration methods has attracted much

attention recently. Attention is now being directed toward developing

criteria to define stability limits for various integration schemes. Efforts

357
have been directed toward general solution procedures that are applicable

to dynamic analyses of assemblages of structural elements exhibiting various

types of nonl i ne ari ti e s . Some of the more widely used numerical integration

methods are: Taylors Series Expansion Method, Newmark & Beta Parameter

Method, Runge Kulta-Gill (4th order) Method, and Linear Acceleration Method.

Details on these methods can be obtained from the References [26, 49, 51, 52]

E . Probabilistic Methods of Seismic Analysis

The type of probabilistic method of seismic analysis to be used is

largely determined by the form in which the loading is specified. Statistical predictions of seismic loadings can be made in terms of probabilistic

seismic coefficients or by defining the ground motions in terms of probabilistic response spectra or stochastic processes. The probabilistic analysis

can be extended to incorporate the uncertainty in the structural characteristics. Probabilistic modeling of the structure almost invariably requires a

s i mu 1 ati on- type of approach, such as Monte Carlo simulation, because analytical solutions become extremely difficult if not impossible. However, the

structures are generally considered deterministic because the structural

uncertainty is relatively small and the probabilistic modeling results in

complications described above.

In the case of probabilistic seismic coefficients or response spectra

and a deterministic structure, the analysis can be performed as described in

the section on deterministic analysis except that the seismic structural

responses are probabilistically determined by appropriate probability com- binations and deviations. If the seismic ground motions are modeled as stochastic processes, the so-called random vibrations approach becomes

necessary. The stochastic processes can be stationary or non-stationary

and are defined by such characteristics as spectral density and correlation

functions, probability distributions and moments, etc. as pertinent. The

characteristics are postulated on the basis of the past ground motion data

and convenience in analysis. The structure is generally modeled as a

1 i near- ti me- i n va ri ant system and the concept of transfer function (which can also be used advantageously in some deterministic analyses) is used to

probabilistically determine the structural response. Nonlinear and timevariant systems can also be analyzed but with increased difficulty and effort.

Details of the random vibration analysis can be found in the literature [2,

16 , 1 7 , 20 , 21 , 35 , 42 , 53]

.

The probabilistic methods of analysis are perhaps better suited for

seismic and other similarly erratic loadings because the uncertainty of the

phenomena is treated explicitly. However, these methods have not been used

extensively, so far, because they are still being formulated and developed

to be useful in specific applications.

III. Review of Structural Configurations and Modeling Procedures

A. Engineered Structures

Building structures may be of many types and configurations, and the

possible approaches to the modeling of such structures are equally varied.

The following text describes a number of the more widely used approaches to

the representation of the physical characteristics (or the modeling) of

building structural systems. The discussion is directed primarily to the

analysis of structures for dynamic loads. Normal static vertical load

analysis, while implicitly included in many cases, is not explicitly discussed.

Prior to the advent of the high-speed digital computer, the engineer was forced to rely on approximate techniques that could be performed either by

358
hand or by means of a desk calculator. Examples of these techniques are the

Cantilever and Portal methods and modifications thereof as discussed in the

previous section. In these cases, certain assumptions are made regarding

the distribution of internal forces within the members of the structure,

which result in reducing a statically indeterminate problem to a statically

determinate one which can be readily solved. Other approaches such as those

based on the slope deflection equations and moment distribution method have

also been employed. These methods, while still used by some engineers, have

largely been precluded by more efficient techniques based on the displacement

(of stiffness) method of analysis and the use of digital computers.

Modern structural dynamic analysis has been made possible by the development of large, high-speed digital computers and by the systematizing of the

calculations in matrix form. The two complementary methods - force (or

flexibility) method and the displacement (or stiffness) method - can readily

be programmed. The latter method is actually more suitable for computer

generalization.

The 1 ateral -force resisting elements of a structure may be one type or

a combination of two basic types - frame (moment resisting, braced, tube),

or shear wall. In the analysis of structures of these types, it is generally

assumed that the floor diaphragms are rigid in their own plane. In most of

the initial efforts, which were directed towards planar structures, it was assumed that the floor slabs only translate and do not rotate. This assumption is valid when the structure is reasonably symmetrical. If the structure

is not symmetrical, then the building must be modeled th ree-d i mens i onal 1 y

.

In this case, the floors are assumed to translate in a horizontal plane and

rotate about a vertical axis. Examples of two- and three-dimensional building

models are shown in Figures 1 and 2, respectively.

A.l Framed Structures

The dynamic analyses of linearly elastic moment-resisting framed

structures and braced frame structures, both planar and three-dimensional,

have been well documented in the literature in the past few years [5, 7, 10,

22, 23, 35, 48, 54, 70, 71, 72, 73, 74, 75]. Many commercial and private

computer programs exist that are capable of analysing these cases. Most

engineers should, as a minimum, have access to programs capable of performing

such analyses. The engineer should satisfy himself as to the validity and

the applicability of any program he uses. Considerable attention has been directed toward the static and dynamic

nonlinear analysis of framed structures in recent years. In numerous cases

it has been observed that the computed elastic response of a structure due

to earthquake ground motions is considerably greater than code design

response, and yet buildings designed for code forces have withstood rather

severe earthquakes. This disparity has been attributed, at least in part,

to the energy absorption provided by deformation of members beyond their

elastic limits. Attempts to more realistically model tin's phenomenon have

generally involved the utilization of el asto-pl asti c , bi-linear, or RambergOsgood moment-curvature functions for the beams and columns of the structure,

and the performance of step-by-step analyses to determine the response of the

structure. Thus a considerable portion of the effort in the development of

nonlinear analysis techniques has been concentrated in better modeling of

members stressed beyond their elastic limits, although axi al -f 1 exural interaction (the so-called P-delta effect) has also been investigated.

The foregoing discussions have pertained primarily to two-dimensional

(planar) structures. Nonlinear analyses of space frames or three dimensional

framed buildings have been somewhat hampered by lack of test data and adequate

theory for yielding of members intersecting at a complex three-dimensional

joint. Work in this area is progressing.

359
77T 77777"

anar Structure Mathematical Model

, FIGURE 1 EXAMPLE TWO-DIMENSIONAL MATHEMATICAL MODEL

3-D Structure Mathematical Model

FIGURE 2 EXAMPLE THREE-DIMENSIONAL MATHEMATICAL MODEL

360
other approaches to reconcile observed behavior during earthquakes have

included the Reserve Energy Technique [4] which provides an approximate

method of correlating observed and predicted damage due to ground motions.

This method, which is based on the concept of an energy balance between input

kinetic energy and energy losses associated with the vibrating structure, is

useful in estimating the total structural resistance capability and the

nature and extent of damage, if any, to various types of structures.

A. 2 Shear Wall Structures

The analysis of buildings with shear walls has received considerable

attention in recent years, and numerous analytical techniques have been

utilized to determine the deformations of and stresses in the walls due to

lateral loads. The state of the art of research in this area was summarized

by Coull & Smith in 1966 [24] and by the American Concrete Institute in

1971 [1]. A brief review of the methods of analysis that have been employed

fol 1 ows .

The most basic or elementary method is to use beam bending theory, which

can be used for a single shear wall without openings that is not connected to

other walls or to frames. When interconnected walls or walls combined with

frames occur, more advanced theories must be used.

One method for treating interconnected (or coupled) shear walls is the

elastic continuum approach, which has received considerable attention and

has been reported in the literature in numerous instances. In brief, this

approach, in its most elementary form, assumes that the elastic properties

of two walls connected by beams at floor levels remain constant and that the

connecting beams can be replaced by a continuous connection of elastic

laminae. The basic limitation of this approach is the restriction on the

geometry of the configuration of the wall system. Only the simplest of cases

can be treated. ACI Committee 442 [1] presented their opinion on the con- tinuum approach when that stated that the continuum approach has received a great deal of attention and would seem to have reached a state where further

development in analytical methods for this approach, while of theoretical

interest, is not likely to result in greatly improved design techniques.

A significant advance in the analysis of buildings with shear walls

came with the development of the high speed digital computer and matrix

techniques of structural analysis. One resulting approach to the analysis

of frames and walls is to use frame analysis programs which have the capability of treating finite joint size. In this approach, the walls are analyzed

as frames, where the connecting elements between walls are treated as beams,

and the walls are treated as equivalent columns. The finite widths of the

equivalent columns are treated by assuming that the beams are infinitely

stiff in the region from the center line of the column to the edge of the

opening. This approach has been described by numerous authors [22, 23, 43].

An alternate approach is to utilize the existing frame analysis programs

which do not have the capability of representing finite joint size. In this

approach, the columns are assumed to be of the same dimensions as the walls

and equivalent members are used to represent the beams. A member of equivalent stiffness is derived to represent a beam with infinitely stiff end regions.

The two computer approaches described above have been termed frame

analogies or wind column analogies and are often used by the practicing

engineer.

Most of the procedures described in the literature for the treatment of

buildings with shear walls are for two-dimensional or planar problems. The

three-dimensional problem has been investigated by investigators [34, 71]

who have sought to determine both the lateral and the torsional stiffness

361

493-810 O - 73 - 24
of the frame and shear wall structure. Examples of these approaches have

included the application of non-uniform torsion theory of thin-walled members

to determine the wall stiffness properties.

The development of the finite element method of structural analysis,

documented in excellent tests [48, 75] has made more detailed investigations

into the behavior of shear walls possible. The finite element method permits

investigation of complex shear wall configurations and does not require the

idealizations and approximations required by the methods previously described.

The application of the finite element method to the analysis of shear walls

has been discussed in numerous instances in the literature [31, 44].

A. 3 Frames with Filler Walls

In addition to shear walls, filler walls (or infilled walls or panels)

or partitions may occur in a structure. Filler walls and partitions are not

generally considered by the structural engineer to carry vertical load. They

often serve only to partition space or to enclose a structure, although in some instances they may be designed to resist lateral loads. A filler wall

usually fills the space created by the beams and columns of a building frame

and is usually in or slightly offset from the plane of the frame. A partition is any nonstructural wall that is used to divide space within a building,

and it may be offset from the frames. The importance of filler walls to

structure response is generally recognized and has been pointed out by several

authors [5 , 9 , 25 , 28 , 30 , 40]

.

Various approaches have been used to model the effects of filler walls

on frames; one such approach is the equivalent brace or strut method. In this method, the wall is replaced by a diagonal strut in the frame. Formulae

based partly on test data and partly on analytical studies to determine the

stiffness of the equivalent strut have been developed [8, 64, 65, 66].

As previously mentioned, the development of the finite element method

has had a considerable impact on the field of structural engineering, and a logical application of the method is to model the effects of filler walls

on the response of building frames. This approach has been utilized and

presents a rational basis for the determination of the dynamic response of

a structure with filler panels [40, 47]. An example of an analytical model

used for a four-story frame structure with filler panels is shown in Figure

3 [40].

B . Non-Engineered Structures —

Building codes presently do not cover the detailed seismic design of all

buildings. The SEAOC Recommended Lateral Force Requirements [62] exclude all

Type V buildings of Group 1 occupancy which are less than twenty-five (25')

in height, and exclude minor accessory buildings. Type V buildings with

Group 1 occupancy include dwellings and lodging houses. The UBC specifies

that all buildings or portions thereof which are constructed in accordance

with the conventional framing requirements specified in Chapter 25 (Wood)

[36] shall be deemed to meet the seismic design requirements.

This approach by codes for dwellings and similar structures is not

surprising. Detailed seismic analysis for each small building is generally

uneconomical because of the design cost for the owner and the extra cost for

building officials to check the calculations. The code requirements for such

structures have usually been derived from empirical experience and generally

have been satisfactory for structures built in the past 15 to 20 years.

Present code requirements and design procedures, however, do not provide

uniform factors of safety because the dynamic response characteristics of

these structures are largely ignored. Ties or anchorage of walls and other

362
FRAME NODAL OeGREBS

OF FRSeoOM

Y' TRANSLATION

X - TRANSLATION

/ROTATION AAOUT i-AYlS

4—^—f 1 COLUMN ELEMSNT

^PANSL ELEMENT

FIGURE 3 FRAME STRUCTURE WITH FILLER PANELS

363
components often are specified based on calculations of typical situations.

Residential damage at Santa Rose in 1969 and at San Fernando in 1971 mostly

occurred to structures older than 15 years, although many new or relatively

new homes in areas of severe ground shaking suffered considerable damage or collapsed during the San Fernando earthquake.

Damage to residences and other small non-engineered structures often

occurs due to exterior walls pulling away from the roof and floors and thus

causing serious stability problems or collapse. Other types of damage

include failure of chimneys, building sliding off of foundation piers, or excessive lateral deformations causing cracking of plaster and breaking of

windows or collapse. In addition to structural damage, many bric-a-brac and

similar items have been damaged by sliding or falling off of shelves or

f urn i ture

.

Observations of many incidences of damage and specific response measurements indicate that the dynamic response characteristics of non-engineered

structures can significantly affect their response to seismic ground motion.

Vibration tests of residential buildings indicate that their fundamental

periods fall in the range of approximately 0.05 to 0.2 seconds [57]. Onestory masonry buildings generally fall in the lower end of this range and

two-story wood frame buildings fall at the higher end. Vibration test data

also show that soi 1 -f oundati on stiffness influences the dynamic response

characteristics of low rise structures.

The detailed consideration of system dynamic response characteristics

in the analysis of such structures would be too costly in nearly all cases.

An approach whereby simplified procedures can be used must be developed.

Such procedures should consider the structure stiffness - whether it is rigid,

semi-rigid, or flexible; the type of construction - all wood, wood and plaster

or stucco, wood and gypboard, or masonry; height of structure - whether one

or two-story; and foundation soil properties - for example, seismic shear

velocity Vg > 3 ,000 to 4 ,000 f ps , Vg >_ 1 ,500 < 3 ,000 fps , or Vg < 1 ,500 fps

(or some other values). Tabulations of structure response characteristics

for each of the above could be developed and appropriate lateral force

criteria specified. Of course, connection details, ties, anchors, etc.

should also be specified.

C . Computer Applications

Numerous computer facilities, methods of communication with computers,

and computer programs are available to the structural engineer today. The

choice of which system to use is perhaps most often based on such factors as: program availability, economics, convenience, and service. This section

presents a brief discussion of various aspects and factors involved in com- puter usage and is directed primarily towards digital computers because of

their wide -spread use and versatility.

C.l Machines and Facilities

There are many types of machines and facilities available, ranging from

the small, relatively inexpensive desk-top computers suitable for office use

to the large, high-speed machines available at the various commercial data

cente rs

.

The small desk-top calculator/computers, which are capable of operation

as a calculator and can be programmed for a limited number of instructions,

have a distinct place in the structural office. They find use in the solution of small problems of a repetitive nature for which it is uneconomical

and inefficient to punch data cards for use on a larger machine. Output

from these calculator/computers is often displayed on cathode ray tubes or 364
may be printed on paper tape. Some machines have capabilities for plotting.

The larger machines are, of course, more suitable for large problems.

Some of the more familiar large computers are the IBM 360 series, the UNIVAC

1108, and the CDC 6400 and 6600. At the top of the list in terms of computational and storage capabilities are perhaps the CDC 7600 and the IBM 360/195.

Certain of the available machines have been developed primarily for business

applications and others have been developed for scientific usage. Machines

of the latter type are generally more satisfactory for engineering applications because of fewer difficulties with numerical precision.

C.2 Communication with Computers

Communication with the smaller calculator/computers is generally by means of a keyboard or paper tape; there are numerous methods and languages

available for communication with the larger machines. These methods can perhaps be categorized as two basic types: conversational and batch.

The conversational mode of communication with computers is generally

implemented by use of typewriter-like, low-speed keyboard terminals. The

terminal may be at the computer center, or may be at the user's office and

linked with the computer center by telephone lines. Using such a keyboard

terminal in a timesharing environment, the engineer can instruct the computer

by means of a specially developed language, such as BASIC or APL. Program

execution can be started, interrupted, or stopped; program statements or values can be altered; and execution can be resumed upon command by the user. Such systems usually include libraries of mathematical and engineering programs and subroutines. The keyboard terminals are suitable for problems

that require more programmed instructions than can be used in the desk-top

type devices, and yet do not require large quantities of input or output or long programs.

Other more elaborate conversational systems utilizing keyboard terminals

have expanded ranges of capabilities - from conversationally developing a program to changing a program already stored - or to simply executing a stored program. Such systems are capable of conversing in the well known

languages such as FORTRAN and COBOL, and some systems permit optional input

or output at either the terminal or data center.

Communication with computers in the conventional batch mode simply means

the input of the program and data by means of cards, tape, or disk. Operational instructions are provided to the system by means of control cards,

and the computer programs themselves are usually in one of the more common languages, such as FORTRAN. This type of service is oriented toward the

processing of large production jobs through a variety of types of high-speed

te rmi nal s

.

The high-speed terminals, which may be installed in the engineer's office

or at a central location within a city, vary in type and complexity. They may

consist of a cathode ray tube and keyboard for command and query purposes,

card reader and/or punch, a high or low-speed printer, plotting capabilities,

tape or disk drives, and a small computer which permits some calculations at

the terminal. Hybrid modes of operation are also possible. A data base may

be developed interactively and executed in a batch processing mode.

C.3 Computer Programs

There are many programs for the static or dynamic analysis of building

structures that are commercially available or that can be obtained from

various sources. Some of the more popular and important of these programs

are

:

365
DYfM - STARDYNE - Static/dynamic structural analysis system for

static, stability, and dynamic analysis of linear elastic

structures

.

EASE - Performs static structural analysis of linear threedimensional systems using beams, membranes and plate structural

el ements

.

ELAS - Finite element program for static three-dimensional

structure analysis

.

FRAN - HOUSEM - Framed structure analysis program for large

three-dimensional frames.

ICES - Integrated civil engineering system supporting, among

othe rs

:

DYNAL - Dynamic structure analysis

STRUDEL - Framed structure analysis

NASTRAN - Static and dynamic analysis of general three-dimensional structures.

SAM IS - Structural analysis and matrix interpretive system

for computing forces, displacements, stresses, mode shapes,

and frequencies of generalized structures.

SAP - Structural analysis program for linear elastic analysis

of three-dimensional structural systems.

S TRESS - Structural engineering system solver for frames,

trusses, culvert sections, etc.

IV. Factors to be Considered

The response of any structure to dynamic ground motion is a complex

phenomenon. Many factors have to be considered - often they are interdependent. A number of important considerations which can affect the

response of a structure are discussed in this section.

A . Damping and Energy Losses

Determination of the damping coefficients to be used is one of the most

important and difficult steps in the seismic analysis. There are relatively

few applicable test data to support an accurate estimate of the true damping

of a structure. Most available test results are based on very small amplitude distortions or on component tests, and the results probably do not

accurately reflect the damping that might be expected for the large amplitude

motions associated with a severe earthquake. And yet, small changes in

assumed damping may significantly change the calculated response of a structural system. It is apparent, therefore, that considerable effort is required

in the research field to provide test data that will minimize the uncertainties in the damping values currently used. The following is a discussion of

several aspects of the damping question. Topics covered are: analytical

conditions, test results, and possibilities for isolation or increased

dampi ng

.

366
A.l Analytical Considerations

The literature on clamping contains frequent reference to the fact that

damping in real structures, especially those of composite materials, is some unknown combination of energy absorption by internal elastic and inelastic

deformation of the materials including foundation soils, and by rubbing or friction between various elements and materials. Damping is no doubt a combination of various forms, such as Coulomb, viscous, etc. The Coulomb or frictional damping force is constant under velocity changes, the viscous

damping force is proportional to velocity, and there may well be quadratic

or other powers of velocity that would best express the real damping forces

developed during building vibration. In spite of these uncertainties it is

customary, and always convenient, to assume that damping is of viscous form.

In the case of more complex multi -degree-of-f reedom systems, the energy

absorption capabilities of a structure are also treated by convenient mathematical techniques. In linear elastic analyses by the modal superposition

method, damping is usually assumed to be simple viscous modal damping. In linear or nonlinear direct integreation analyses, damping is often assumed

to be proportional to the mass and/or stiffness of the structure. The validity of this latter approach is questioned by many investigators. In the

direct integration case, other procedures have also been utilized to rationally determine the damping matrix [40].

All of the above techniques account for damping in an overall sense. No detailed attempt is made to specifically define the distribution of the energy

absorbing capabilities. The energy absorbing capabilities of a structure

are generally a function of the types of construction and materials used for

structural members and for nonstructural walls and partitions. A structure

that is relatively brittle will have little strength beyond its elastic limit

and hence little capability to absorb energy and resist overload. On the

other hand, a relatively ductile structure that can withstand numerous cycles

of deformation into the inelastic range will have considerable energy absorption capacity.

These concepts led to the development of moment-resisting ductile

concrete [13] and steel frames. The basic idea is to ensure the capability

of the structures to deform past the yield limit. It is important to execute

the design so that any yielding will initially occur in the beams or girders

to preclude possible instability of the columns. In using this approach, the

energy absorption capabilities of the structural frame are, in a rough sense,

distributed in proportion to the energy absorption capability of the individual members. The distribution of energy absorbing components can significantly effect the response of a structure. For example, shear walls

supported by columns can produce discontinuities which result in extremely

high forces and stresses. As another example, earthquake motions can produce

high torsional motions in unsymmetri cal buildings. Nonstructural partitions

and panels can contribute to the energy absorption capability of a structure,

although the degree of contribution may be small in modern structures. These

energy absorption capabilities may be distributed throughout the structure.

The total kinetic energy of a vibrating system may be equated to heat

gain, stored or potential energy, and work done. Although linear systems

are usually assumed, many systems are not truly linear for various reasons including hysteresis, yielding, cracking, deteri ori ati on , failing, etc.

For inelastic analyses, the concept of equivalent damping [37] is useful

in that the inelastic work is combined with hysteresis to provide a measure

of effective damping for possible inelastic excursions. However, this leads

to complications including variable values under different cycles and to errors under large nonlinear deformations.

The Reserve Energy Technique [4, 13] is a concept of analysis under

which the damping is confined to hysteresis or heat loss with no deterioration

367
or distress to the structure under repeated cycles; i.e, the damping is

restricted to what would normally be considered linear response. The work

done in the inelastic range plus the stored energy is considered to be

represented by the area under the force-deformation curve, adjusted for any

deterioration under repeated cycles. Thus non-damaging hysteresis and

damping inelastic excursions can be kept separate in design or analysis.

A. 2 Test Results

Damping values are generally determined experimentally by one or more of

several available means; steady state forced vibration or near-steady state

forced vibration, various means of inducing free vibration such as pull and

release tests, and the spectral response reconciliation method [6]. The

assumption is usually made that the effects are the same as if the damping

were truly viscous and the damping force is proportional to the relative

velocity of a si ngl e-degree-of -freedom system. There may be some error asso- ciated with these assumptions, but there are distinct advantages to the

simplicity and tendency toward standardization of this approach.

Other considerations affecting the damping ratio are the variations with

amplitude or stress level, and variation with the prior history of motion.

Comparisons should consider these parameters, when and if data are available.

A comprehensive review of test results reported in the literature and

other services was performed [11]. This work included the compilation of

data on 171 structure types or components for which there were a total of

409 damping determinations. Of these, there were data on 74 bu i 1 di ngs ( 230

damping determinations). The results of this compilation for buildings is

shown in histogram form on Figure 4. It would be expected that the damping

ratios would increase in many cases under greater amplitude of motion than

that for the tests reported. In general, the higher damping values are associated with composite construction of many materials and elements, and

with greater amplitudes. The low damping values are generally associated

with non- compos i te and more simple construction, and with lower amplitudes.

The prior history of motion is also an important parameter in reinforced

concrete framing and composite construction.

One factor that seriously complicates the damping problem is that

results of various tests suggest tTiat damping in a complex structure is

dependent on both relative and absolute velocity, displacement, and frequency. The fact that damping may be displacement-dependent is particularly

vexing because this tends to cast doubt on the results of all low amplitude

vibration tests as far as damping is concerned.

In order to study the effect of displacement on damping as well as to

obtain other basic data, two full-scale, four-story reinforced concrete

frame test structures were constructed at the AEC Nevada Test Site near Las

Vegas, Nevada. These structures were designed and tested by John A. Blume & Associates Research Division for the Structural Response Program, Office of

Effects Evaluation, Nevbda Operations Office, United States Atomic Energy

Commission [9, 30, 50]. Some tentative results of this testing program are shown in Figure 5.

For these four-story structures, partitions of various types were installed, subjected to ground motions, and then removed. In general, damping is higher for the structures with frame and partitions than for the

frame alone (Figures 6 and 7). However, it has also been observed that the

damping can decrease in a structure with frame and panels, after the structure has been subjected to numerous cycles of ground motions. This can be

attributed, at least in part, to the destruction of the bond or connection

between frame and panel and subsequent ineffectiveness of the panel in the

dissipation of energy.

368
6>9

36

MED/AA/

AVERAGE

51 52

24

2.5

5.1

2 3 4 5 <i 7 6 9

PERCENT OF CRITICAL DAMPING

to II 12 15

FIGURE 4 HISTOGRAM OF 230 DAMPING DETERMINATIONS FOR BUILDINGS

369
c

OJE.

aio

tao.

</) D

o

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0

_ WTTt

- I

1

1 WALLBOARD

PARTITIONS

Unce rtain

'

/

1 ^-1

—1

r

1

/

!

—t

/

oy

<

1

1 _ WITH . 1 WALLBOARD

PARTITIONS

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o^

?

H 234 5601 234

Percent of critical damping

S Dir. , North Struct. E-W Dir., North Struct.

Oin T3

O

0.18

0.16

0.14

0.12

0.10

0.08

0.06

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PARTITIONS

/j I.

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• 012345601 234

Percent of critical damping

N-S Dir., South Struct. E-W Dir. , South Struct,

FIGURE 5 DISPLACEMENT VERSUS PERCENT OF CRITICAL DAMPING

370
FIGURE 6 EXPERIMENTALLY OBTAINED FUNDAMENTAL MODE DAMPING RATIOS

FOR LONGITUDINAL MOTIONS

371
FIGURE 7 EXPERIMENTALLY OBTAINED FUNDAMENTAL MODE DAMPING RATIOS

FOR TRANSVERSE MOTIONS
studies of data recorded from the San Fernando Earthquake for several

high rise structures indicate that damping can vary considerably [12]. For

structures that responded within the elastic range, some low damping values were obtained - on the order of 1% to 3% (of critical viscous damping). For

two similar concrete structures, which were excited to roughly the same levels, quite different damping results were obtained. For one structure,

damping of approximately 5% to 10% was obtained, whereas damping of 2% to 5% was obtained for the other structure. Both of these structures showed a marked change in response characteristics after about six seconds of earthquake induced motion - which indicates a change in structural properties

possibly due to excursions into the inelastic range or due to failure of the

connections between partitions and the structure.

Tests have also been performed on a series of one-story and two-story

houses [57]. These houses, located in Colorado, were constructed of adobe,

wood frame, or concrete block, and were generally in poor condition in that

their structural integrity was not comparable with modern construction. The

tests performed on these structures were of the pull-release type, in which

a static lateral force was applied on each building near the eave line to

deflect the building. The force was then released and the resulting free

vibration motion recorded. The experimentally determined fundamental periods

of vibration of these structures were in the range of 0.09 to 0.20 seconds.

The damping in these structures was determined from the decay rate of the

recorded motion. Values obtained are summarized in Table 1. The data

indicate an average damping of from 5% to 6% for these houses.

It can be concluded from a review of past test results that data to

adequately define the magnitude and mechanism of damping are still definitely

needed. Until these data become available, the engineer is forced to use his

best judgment in the selection of damping values. Conservative values should

be selected to ensure satisfactory and safe performance of the structure.

Table 1

OBSERVED BUILDING-AS-A-UNIT MODAL DAMPING

Structure

Number Description

Test Damping (Percent of Critical

Direction Variation Average

One-Story Adobe House

Two-Story Frame House

One-Story Frame House

One-Story Concrete

House

Free-standing Chimney

Transverse

Transverse

Longitudinal

Longitudinal

Longitudinal

4 - 7

4 - 10

0 - 30

**

2.5 - 4.0

5.7

6.0

*

5**

3.0

* Approximately 30% damping observed in first cycle, thereafter damping

was 0 - 2%

** Approximated -- record too noisy to obtain damping for individual peaks

373
A. 3 Isolation and Increased Damping

Numerous ideas have been set forth by various authors to isolate a structure from the earthquake-induced ground motions. These ideas have

generally involved the concept of increasing the fundamental period of the

structure to the point where the structure is so flexible that its response

to the ground motions is extremely small. One method by whi.ch this can be

accomplished is by the use of the so-called flexible first story, where the

columns of the first story are relatively flexible [27, 33, 45]. Another

approach, which has been employed on several recent buildings, is to "hang"

the structure from a central core (or cores) by means of cable systems, thus

greatly increasing the flexibility of the system. Other proposed methods are

to use mechanical devices, such as rollers [18], or to use flexible materials

between column base plates and the foundation system.

Various schemes have been proposed to increase energy absorption capabilities of a structure by mechanical means. These schemes have generally

involved the use of physical dashpots (viscous dampers) or the use of

frictional dampers. These are sometimes proposed for use in conjunction

with the mechanical isolation devices mentioned previously. The use of

mechanical damping (and isolation) devices has one disadvantage that is often

cited; the requirement for periodic maintenance during the life of the

structure - and this maintenance could be easily forgotten after a few years

or after a change in ownership of the building.

The use of isolation devices and the various means of increased damping

is an intriguing concept and certainly warrants future research. As of yet,

the full effectiveness of this approach and the practicability of such devices

has not been demonstrated. However, the effects of these types of devices on the dynamic response of structures can be determined by employing advanced

analytical techniques. A series of proper analytical studies, which include

vertical and horizontal motion response, etc., coupled with detailed designs

of such devices, would be a significant contribution to the field of earthquake engineering. The results should be confirmed with vibration testing of

models that are as large as possible.

B . Effects of Nonstructural Elements

The walls of a building may be of many types and configurations. They

may be part of the engineering design of the structure or they may be architectural or nonstructural in nature. In many cases, the walls of a structure

are included in the design of the building to resist lateral forces induced

by wind or earthquake. Such walls are generally called shear walls and may

also serve a load bearing function. Other types of walls are called filler

walls (or infilled walls or panels), or partitions. These walls are usually

constructed of reinforced or unreinforced concrete block, clay tile, brick,

wood or metal, and are not designed as an integral part of the structural

system of the building.

Filler walls and partitions, even in the cases where they are nonstructural can have a considerable influence on the lateral response of a structure.

Investigations of high-rise buildings and four-story test structures [8, 9,

30, 40] have shown that the presence of filler walls and partitions must be

considered in the analyses of structures for lateral response. In these

investigations, it has been shown that the presence of filler panels can have

a significant effect on the frequency characteristics and dynamic response of

a structure.

Investigations of response data from high rise structures from the San

Fernando earthquake have shown marked changes in the response characteristics

of structures at certain times during the response. This can be partially

attributed to the changing characteristics of partition-structure interaction

374
during the response time -hi story.

One particularly vexing problem that often occurs with filler walls and

partitions is that they may have only limited strength relative to the frame

of the structure. That is, they may fail at some point in the loading time

history, and the frames and any remaining walls must then resist the entire

lateral force. An additional problem -- sometimes created by lax construction

practices and techniques -- is incomplete connection between the wall and

frame. In these cases, mortar or grout may be of low strength and reinforcement, if any exists, may not provide continuity between wall and frame. Also,

prior motion of the building due to wind or ground motions may have damaged

the wall to the extent that the connections between the wall and frame are partially destroyed. These effects have been shown to be quite important in

determining the response of a structure [40].

In addition to nonstructural partitions and filler panels, exterior precast concrete fascia panels can significantly effect the response of a structure. These panels may in come cases be more significant than interior

partitions. They are often designed with sliding or deformable joints so that they do not inhibit the movement of the frame structure. However, if

the joints are not properly designed, or if they should bind during seismic

motions, the pre-cast walls may greatly increase the stiffness of the structure. This can result in increased seismic forces imparted to the structure

or a redistribution of forces, which in turn could decrease the safety of the

structure. In addition, the panels or connections themselves could fail,

which could present a serious hazard to any persons or property in the areas adjacent to the building [63b].

It should be pointed out that damage to partitions and exterior panels

and glass can represent a serious economic loss, since these items may represent a significant portion of the total cost of the structure, and, in fact,

may cost more than the structural frame.

C . Site Soil Effects on Structure Response

The term soi 1 -structure interaction is often used to designate a number

of different phenomena; modification of the site ground motion, interaction

of the soi 1 -structure system, or effect of foundation soil deformation due to

dynamic loading on building response. The modification of site ground motion

was covered under the review article Earthquake Hazards for Buildings. The

other effects will now be examined.

Since most structures are supported on soil or rock formations and since

ground motion waves are transmitted to structures through the underlying

foundation materials, it is reasonable to assume that structure response to

ground motion is affected by local soil or rock conditions. Numerous observations concerning the influence of local soil conditions on structure damage

caused by earthquakes have been documented - starting with the 1906 San

Francisco earthquake. In addition, some pertinent information is available

from studies made during underground nuclear explosion experiments.

From studies of these observations, it is possible to identify several

important factors whicti have a direct effect on structure response and are affected by local soil conditions:

• Site ground motion modification (Review article. Earthquake

Hazards for Buildings),

• Stiffness characteristics of foundation soils,

• Dynamic response characteristics of structure.

375
• Dynamic soil consolidation potential,

• Liquefaction potential of site soils, and

• SI ope stabi 1 i ty

.

The effects of stiffness characteristics of foundation soils were noted

as early as the 1906 San Francisco earthquake. Many buildings on bay fill

suffered much more extensive damage than those on hard rock. At Kanto, Japan,

in 1923, rigid buildings founded on rock were more severely damaged than those

on deep alluvium. In the 1957 San Francisco earthquake, local soil conditions

both amplified as well as attenuated ground motions.

The dynamic response characteristics of structures appear to be affected

by the site soil conditions. As noted at Kanto, rigid buildings on hard rock were damaged more severely than those founded on alluvium. This could be

explained on the basis that the ground motion intensity in the alluvium was less for periods coincident with the periods of the rigid buildings on alluvium than for rigid buildings on rock. It is also possible that the

soi 1 -structure system damping was greater in the softer soils.

Recent studies by Scholl [56], Seed [59] and others lend credence to

another explanation - the structure response is dependent on the relative

periods of the soil and structure, and on the damping of the so i

1 -structure

system. Observations of response to ground motion induced by underground

nuclear detonations by Blume [7] and Scholl [57] substantiate this hypothesis.

Figure 8 is a comparison of response spectra calculated for motions recorded

free field and in the basement of high-rise structures. In these cases the

stiffnesses of the sites were greatly different than those on the buildings.

It is of interest to note that the Imperial Hotel in Tokyo designed by Frank

Lloyd Wright and completed in 1921, was designed to "float" during an earthquake; the building was much stiffer than the underlying soils. The hotel

withstood the 1923 Kanto earthquake with essentially no damage. There are also a number of theoretical studies of soi 1 -structure response [32, 38, 56,

69]. In summary, there are empirical evidence and theoretical considerations

to justify the following conclusions:

• Local soil conditions do effect the dynamic response of structures,

• The amount of the effect is primarily a function of the frequency

content of the ground motion and the relative stiffnesses of the

soi 1 and structure ,

e In many cases local soil conditions may be more important to low

rise buildings than for high rise structures.

Three other effects of site soil conditions should also be briefly

considered: the potential for dynamic soil consolidation, liquefaction, and

slope stability. Considerable damage has been caused by these factors; San

Fernando (1971), Niigata, Japan (1963), and Fukui , Japan (1948) produced

numerous examples of damage.

Observations of dynamic soil consolidation caused by earthquake vibratory

loads have been made on numerous occasions. In addition, vibratory loading is

used to improve building sites by compacting loosely consolidated sands. The

results of field work and field and lab testing show that the degree of con- solidation is a function of loading intensity, frequency and duration, and

of the characteristics of the soil, i.e., whether it is cohesionless or cohesive soil. The consolidation of cohesionless soils is also a function

of their relative density and the confining pressure.

The effects of liquefaction were perhaps most spectacular at Niigata.

Liquefaction is a "quick" or liquid condition developed in cohesionless soils

376
STATION m

BASEMENT

-

, /SLOG. A

:' \ , LONeiTUOINAL

STATIONm .

~~ SASBMBNT

LONQITUOINAL

BASEMENT

/SLOG. A

TRANSYEf^Se

STATION I N/5 BASEMENT

BLOe. F

TIZANSUllSe

STATION IPAN\P/N6 5%

0.1 1.0 PERIOD IN SECONDS

FIGURE 8 COMPARISON OF BASEMENT AND "FREE FIELD" RESPONSE SPECTRA, EVENT BENHAM

377

493-610 O - 73 - 25
under cyclic loading - excess pore pressure decreases the intergranul ar pressure to the point where the material has zero strength. Field tests by

Florin and Ivanor [29] showed that liquefaction could be produced by blasting.

Seed and Idriss [60] and Kishida [39] described methods for evaluating liquefaction potential. The required conditions include:

• Cohesionless soil, uniformly graded - sand is more susceptible to

liquefaction than gravel,

• Relative density - loose sands have higher liquefaction potential

than dense sands

,

• Initial confining pressure - liquefaction potential decreases with

increasing confining pressure,

• Ground motion intensity - liquefaction potential increases with

increasing intensity,

• Duration of ground motion - liquefaction potential increases with

increased duration,

• Depth of water table - liquefaction potential decreases with

increased depth to water table.

Another factor that should be evaluated is the potential for si ope

i nstabi 1 i ty . The Turnagain Heights failure (Alaska, 1 964 ) [61] is perhaps

one of the most spectacular. Slope failures also occurred at San Fernando

and in other earthquakes [58]. Approximate quantitative evaluations of

slope stability can be made based on proper soils engineering investigation,

experience and judgment.

D . Vertical Acceleration Effects

Vertical accelerations currently are given little or no consideration in

the design of conventional structures. However, there may be instances where

vertical accelerations could contribute to the failure of a structure under

seismic conditions. Vertical acce 1-erati ons could contribute to the compressive failure of a column, reduce the factor of safety against overturning, or cause a critical condition in prestressed beams by reducing the effective dead

load on the beam. Vertical motions have also contributed to the collapse or severe damage to marquees or entrance canopies.

Investigations of the acceleration - time history records of a number of

recorded earthquakes [41] have shown that peak vertical accelerations ranged

from 0.36 to 0.67 times the peak horizontal accelerations. The Parkfield

earthquake was of particular interest in this respect as accelerations were recorded at varying distances from the San Andreas fault trace. The vertical

and horizontal accelerations decreased rapidly as the distance from the fault

increased. This rapid decrease in accelerations is important because accelerograms for the other major earthquakes have been recorded at distances from 7 to 30 miles from the fault trace. Thus structures close to the moving fault

or epicenter may be subjected to higher ground motions than indicated by most

records. It appears that vertical accelerations close to the fault may be

much higher than those shown on available records. This observation is also

confirmed by experience with underground nuclear explosions. In this case,

for locations relatively close to the point of detonation, vertical accelerations are a significant percent of the horizontal accelerations, and in some cases exceed the horizontal accelerations.

Investigation of the horizontal and vertical records for the Olympia,

Taft, and Golden Gate Park earthquakes indicate that while the peak vertical

accelerations may not occur at exactly the same time as the peak horizontal

378
accelerations, major vertical motions do occur within the same general time

as the major horizontal accelerations. Thus the vertical and horizontal

accelerations could be conservatively assumed to act simultaneously. This

assumption is generally applied in the earthquake design of nuclear pov/er

plants and other critical structures.

Si ngle-degree-of-f reedom response acceleration spectra computed from the

vertical acceleration time-history records for the Olympia, Taft, El Centre,

and Golden Gate Park earthquakes show significant amplification in the low

period (or high frequency) range [41]. The spectra from Taft show amplification in the period range 0.05 to 0.5 seconds, and all of the vertical

spectra obtained show amplification in the 0.05 to 0.25 range. The El Centro

spectra show a definite attenuation of the long period components. It is

extremely difficult to generalize as to the shape of the vertical spectra

versus the shape of the horizontal spectra on the basis of the few records

investigated. It is reasonable to assume that the shape will be a function

of distance to the fault, type of fault, intervening geologic structure, soil

type, etc. In general, the vertical spectra show an attenuation of the long

period components of the earthquake records.

Numerous analyses of structures in the vertical direction have been

performed to determine their natural frequencies, mode shapes, and dynamic

response [41]. In almost all cases it was found that buildings are not rigid

in the vertical direction as is commonly assumed. They are, of course, much

stiffer in the vertical direction than in the horizontal direction. As an example, the fundamental period of vibration in the vertical direction of a 51-story structure was calculated to be 0.60 sec., and for a 24-story building was 0.32 sec. It is important to note that the vertical periods were within the range of amplification indicated by the response spectra developed

from the vertical earthquake records.

The dynamic response of the structures to input vertical motion was also

determined. It was found that the peak vertical acceleration at the top of

the 24-story structure was 0.42g for the Taft earthquake, which is 3.4 times

the peak ground acceleration. The peak vertical acceleration at the top of

the 51-story structure was 0.38g for the same earthquake, which is 3.1 times

the peak ground motion. It should be emphasized that these amplifications

are due to axial deformations of columns only. No response of beams or slabs

was included.

Examination of records obtained during the 1957 San Francisco and 1971

San Fernando earthquake bear out the above results. Amplifications vertical

accelerations were recorded during the 1957 San Francisco event varying from

3 at the 15th floor of one building to 5.67 at the 14th floor of another

building [17a].

During the San Fernando earthquake two nearly identical seven-story

reinforced concrete buildings [12] had peak recorded vertical accelerations

of 0.18g and 0.086g at the ground floor and 0.24g and 0.140g respectively at

the eighth level (roof). A twelve-story reinforced concrete moment resisting

frame building had O.lOBg peak vertical acceleration at ground level versus 0.150g at the 13th level (roof). For these three structures the amplification

of vertical motion was in the order of 1.5. However, a 21-story reinforced

concrete moment resisting frame had peak vertical accelerations of 0.087g at

ground floor and 0.260g at the 20th floor - an amplification of 3. Similarly,

a 32-story moment resisting steel frame building showed an amplification of

3.8 (0.059g at ground floor and 0.224g at roof). It is therefore evident

that significant magnification of vertical accelerations does occur in structures. The commonly held opinion that structures are rigid in the vertical

direction is not valid; they are very stiff but are not rigid.

In conclusion, it should be noted tliat amplification of vertical ground

motion in a structure may contribute to the failure or overstress of a structure under seismic conditions. Vertical accelerations could increase or decrease the load on columns during an earthquake by 20'b or more. This may

379
be critical when combined with the loads from the lateral response of the

structure. Vertical accelerations therefore could contribute to the com- pressive failure of a column or reduce the factor of safety against overturning.

E . Effects of Aging and Prior Loadings of Materials

Aging of some materials can increase their strength up to a point - concrete is an example. Conversely, concrete or masonry structure strength

can be reduced by cracking due to shrinkage, thermal cycles or differential

settlement. Steel frames, on the other hand, should maintain a relatively

constant strength. However, the usual greater flexibility of steel frames

may make their exterior and interior walls more susceptible to damage from

dynami c 1 oadi ngs

.

Prior loading of a structure can in some cases significantly affect its

response. It has been observed that if a concrete structure has been deformed

beyond the yield limit, a significant change in period of vibration can result.

This has been attributed to cracking of concrete in the tension zones and the

resulting more flexible cracked sections. Interestingly enough, it has been

determined in certain test structures that the fundamental period of vibration

of a structure, after lengthening over a period of time due to loading, has

actually decreased after a period during which the structure was not loaded.

This could perhaps be attributed to "cementing" of the cracks due to dust,

wi nd , and rai n .

A structure with filler panels or partitions may have quite different

physical characteri sti cs 'before and after loading. The panels or partitions

may have only limited strength relative to the frame of the structure. That

is, they may fail at some point in the loading time history, and the frames

and remaining panels and partitions must then resist the entire lateral force.

This phenomenon can represent a significant change in the physical properties

and corresponding response characteristics of a structure [12].

F . Effects of Variations in Parameters

Regardless of what analytical techniques or procedures are used in the

seismic analysis and design of structures, it should be recognized that none

of the procedures are exact. Uncertainties always exist. These uncertainties,

which are in the parameters used in the analyses, can be categorized as input

motion, mass, damping, and stiffness - all of which can influence the dynamic

response of a structure.

Uncertainties in the input motion can in part be treated by the use of

relatively smooth response spectra for a site, as opposed to the rough or jagged spectra from historic earthquakes. The use of the historic time history records may result in significant changes in response resulting from

small changes in structure frequency. Therefore, if time history analyses

are performed, time histories should be developed which produce relatively

smooth spectra that closely match the spectra postulated for the site.

Procedures are currently in use to develop such time histories [46, 68]. The

range of effects of local soil conditions on the input motion should also be

considered.

The mass of the structure is perhaps the parameter that can be most

accurately estimated. A reasonable variation from the calculated values

might be plus or minus ten percent.

As previously discussed, the damping in a structure is subject to much

conjecture and discussion. However, one point remains clear: the damping

values should be selected conservatively.

380
Variations in the stiffness of the structure v/ill be a function of the

complexity of the structure and the modeling techniques used. The stiffness

of a simple framed structure can be determined quite accurately, whereas it

may be very difficult to determine the stiffness of a complex frame or shear

wall structure. Generally, the stiffness of a structural system is assumed

to be constant with time for the specified loading. In cases where material

or geometric nonl ineari ties exist, this assumption is not justified and may

lead to inaccurate results. Some sources of nonlinearity which produce a change in stiffness are

• Once a structure reaches the yield stress level, the stiffness to

resist further loading is greatly reduced.

• Once a structure undergoes large displacements, linear theory based

on the assumption of small displacements is not valid. As a con- sequence, loads acting on the structure should be applied to the

deformed structure. In building structures, this phenomenon is often

referred to as the P-delta effect, and is often interpreted as reduction of stiffness.

• Another factor influencing the stiffness of a structure is the

contribution of the non-structural components (partitions, windows,

etc . ) .

• The nonlinear stiffness properties of the soil materials underlying

the structure may influence the overall structure response.

In addition to the uncertainties in the analysis, the site soil effects

as previously mentioned may be significant in the design of relatively stiff

structures. For tall flexible structures the soil effects often are less

significant.

The designer must recognize the above possible sources of uncertainties

in the analyses and must incorporate in his design the effects of these

uncertainties on the response of the structure. Seismic analysis is not an analysis with one set of input parameters, but requires a combination of

analysis and experienced engineering judgment.

G. P-Delta Effects

P-Delta effects are generally defined as those effects that are attributed to changes in physical characteristics when a structure is subjected

to large displacements. For linear elastic structures which undergo small

displacements, all loads can be applied to the undeformed structure without

introducing significant errors. With large displacements, it is necessary

to apply the loads to the deformed structure. For a typical rectangular

building frame, the gravity loads have no effect on the overturning moment

of the undeformed structure. In the deformed structure, however, the over- turning effects due to the gravity loads become increasingly more important

as the story displacements increase. Because such large displacements are generally not possible without at least some members undergoing inelastic

deformations, the P-delta effect will generally be negligible for buildings

responding in the linear elastic range.

A formulation of the P-delta effect, which is sufficiently accurate for

most practical purposes [15] can be derived for a typical column element in

the displaced configuration, which leads to the so-called geometric stiffness

matrix. In this stiffness matrix, certain terms associated with the axial

force in the member have a reduction effect on the member stiffnesses.

The essential features of the P-delta effects may be summarized as follows: As a consequence of the geometric element stiffnesses, the total

structure stiffness is reduced. With increasing axial column forces and

381
increasing displacements, the frame may become unstable and buckling may

result. Moreover, with increasing axial forces, the yield capacities of

column members are reduced, resulting in much earlier formation of plastic

hinges and hence further weakening of the structure.

Without further elastic and inelastic parametric studies, it is difficult

to predict the net change on the dynamic behavior of a structure that might

result if the P-delta effect is considered. The relative significance of many

P-delta effects have not yet been assessed, either in respect to each other or

in respect to other response parameters.

H. Drift Control

It is often impractical and ineffective to attempt to control the seismic

performance of a structure primarily by regulation of stress. Equally, if not more important is distortion control, which is essential to preventing both

structural and nonstructural earthquake damage. If distortion is adequately

controlled, then ductility requirements are usually satisfied, P-delta effects

become insignificant, and damage potential is reduced considerably.

Distortion is generally measured in terms of interstory drift coefficients. These coefficients are defined as the maximum relative lateral

movements between adjacent floor levels divided by the appropriate story

height; interstory drift limitations are seldom specified in static codes.

In codes where they are referenced, drift limitations ranging from 0.002 to

0.005 are usually specified. Whatever drift limitation is set, it should, of

course, be consistent with the level of the prescribed forces. One reason

that drift control has not been defined in codes is that suitable envelopes

of design earthquakes and resulting responses have not been defined. Code

lateral forces greatly underestimate the level of forces anticipated during

a great earthquake. Therefore, it is important to predict the order of real

distortions during periods of strong ground shaking. In lieu of computing

the dynamic response of a structure to gain such knowledge, it has been

proposed that another static force formula be devised to represent a more realistic force level that might be anticipated during a great earthquake.

Such a formula could be used for drift computations, assuming an elastic

response in all cases

.

The need for drift control is considerably greater in structures that

rely on inelastic energy dissipation to survive a major earthquake. Many

structures in existence today fall into this category. During an earthquake,

energy is imparted to a structure, the amount of which is both dependent on the characteristics of the ground shaking and of the structure itself. If

an earthquake delivers more energy to a structure than the structure can absorb and ultimately dissipate, the structure will collapse. Without adequate drift control, certain elements within a structure may be called upon

to dissipate considerably more energy ihan other elements even though such

other elements may have equal or greater energy dissipation capacities. If

the energy demand is too great for these heavily loaded elements and the

other elements cannot share or relieve the overload because of geometric or stiffness constraints, then some form of failure will result. The application of consistent uniform drift criteria generally results in energy being

dissipated uniformly throughout a structure, and more elements share in the

energy dissipation process. Also, ductility demands on any one element are considerably reduced.

Inelastic drift predictions have been generally based on extrapolations

of linearly elastic drift computations and on the non-linear dynamic responses

of simple idealized structures. However, it is now possible to calculate

inelastic drifts directly using recently developed computer software. Limited

studies on several structural systems have indicated that inelastic drifts are,

in general, less than those predicted for identical linear elastic systems.

382
I . other Code Considerations

Code approaches generally include other considerations such as provisions

for horizontal torsion, overturning moments, and special lateral force requirements on parts or portions of structures.

Overall or interstory planwise rotations of a structure result in additional lateral forces that are induced in the lateral force resisting elements.

This twisting, or torsional, motion is attributed to non-coincidence of the

centers of mass and rigidity of the stories of the structure as a whole. Most

present-day codes now require that such torsion be analyzed and the resulting

torsional forces distributed to lateral force resisting elements in proportion

to their rigidities. Frequently, a minimum torsional eccentricity is specified.

The overturning moments as computed by some code formulae include a reduction factor that supposedly accounts for higher mode participation.

Recent studies and events have led to the elimination of this reduction

factor in many codes. Incremental changes in the prescribed overturning

moment at any level are generally required to be distributed to the various

resi sting el ements

.

Certain portions or elements within a structure are often exposed to

much larger force levels during an earthquake than the force level associated

with the structure as a whole because a resonance condition between the basic

structure and such elements is induced. Whiplash at the top of a structure

is evidence of this phenomenon. Parapets, appendages, partitions, chimneys,

tanks, equipment, etc., are generally classified as requiring special con- sideration under seismic loads. A more comprehensive discussion of equipment

analysis is given in Section V.

J . Earthquake Recording Instrumentation

Awareness of the need to have more and higher quality data on actual

earthquake ground and building motions has resulted in the requirement of

several codes that strong motion seismographs be installed within buildings.

By analyzing records of building response to strong ground shaking, seismologists, engineers, and code-writers can gain a more meaningful understanding

of the seismic problem as related to structural response.

V. Equipment Supported in Structures

Equipment supported in building structures may consist of conventional

mechanical and electrical equipment - heating and air conditioning, lighting

and control panels - or it may consist of more exotic and valuable items

such as computer systems. Presently, little or no consideration is given to

the seismic design of such equipment. Conventional codes, such as the UBC,

can be construed to apply to equipment by using the horizontal force factor,

Cp, for ". . . parts or portions of buildings . . although no specific

mention of equipment is made. Thus, this portion of the seismic design is

left solely to the discretion of the individual designer.

If no provisions are made for seismic resistance of the equipment, then

the equipment may be damaged or rendered inoperable if an earthquake should

occur. This could result in injury or loss of life, or an economic loss due

to i noperabi 1 i ty of the facility and the costs of repairs. Examples of damage

to equipment resulting from the San Fernando earthquake are shown in Figures

9, 10 and 11. Much was learned regarding equipment design from the San

Fernando earthquake [63]:

383
•^^ FIGURE 9, DAMAGE TO A PENDANTTYPE LIGHT FIXTURE. ALTHOUGH

HORIZONTAL MOTION WAS UNDOUBTEDLY SEVERE, VERTICAL EARTHQUAKE FORCES MAY HAVE PLAYED A SIGNIFICANT PART IN CAUSING

THIS FIXTURE TO PULL AWAY FROM

ITS SUPPORTING BRACKET. MANY

LIGHTING FIXTURES ATTACHED TO

SUSPENDED CEILINGS OR HUNG ON

RODS OR WIRES FROM FLOORS AND

ROOFS WERE OBSERVED DAMAGED.

DAMAGE NOT ONLY RESULTED IN MALFUNCTION OF THE FIXTURES

BUT ALSO PRESENTED A HAZARD TO

PERSONNEL.

FIGURE 10. THIS SUSPENDED

CEILING, LOCATED IN THE CONTROL

ROOM OF A LARGE ELECTRICAL

POWER FACILITY, COLLAPSED

DURING THE EARTHQUAKE, INJURING

ONE MAN. WHILE LITTLE DAMAGE

TO CONTROL CONSOLES AND EQUIPMENT WAS REPORTED, THE FALLEN

CEILING HINDERED EMERGENCY

OPERATIONS .

FIGURE 1 1 . DURING THE EARTHQUAKE THESE BOILER CONTROL

PANELS WERE DAMAGED WHEN

STRUCK BY SLIDING BOILER UNITS.

OTHER PANELS, INSTALLED WITHOUT ADEQUATE ANCHORAGE, OVERTURNED OR SLID ON THEIR

FOUNDATIONS

.

384
• Mechanical and electrical equipment, and their supports and

connections, must be designed to withstand realistic seismic

forces if sliding or overturning of equipment, disruption of

utility service, or other possible costly damage is to be

avoided or minimized.

• Suspended ceilings, light fixtures, and other "non-structural"

elements must be designed to resist seismic forces without

collapse.

t Proper seismic design should consider the simultaneous effects of

both horizontal and vertical earthquake forces.

• Elevator equipment and emergency power supply systems should be

given particularly detailed treatment in seismic design.

• For equipment located in structures, the possibility of damage

resulting from the interaction of the structure and equipment

should be considered. Components of large diameter piping systems,

especially pipe hangers, that cannot accept the loads imposed on them as a result of interstory building displacements are particularly vulnerable to this type of damage.

- • Equipment having supporting members and connections of brittle

materials, such as insulating porcelain, are especially susceptible

to damage.

From the above, it is evident that equipment supported in a structure

must receive a rational seismic design. One approach to the seismic design

is to estimate the peak horizontal acceleration of the floor on which the

equipment is suspended and then design the equipment for this acceleration

applied as a static coefficient. This approach is valid provided that the means used to estimate the building acceleration is rational, and provided

that the item of equipment itself is rigid, that is, it does not amplify the

floor motion due to its own dynamic response.

Where the equipment is flexible and is supported by a flexible structure,

the equipment-structure interaction becomes a critical factor and must be

considered to ensure an adequate design. For a solution to this problem, it

is desirable to draw on procedures developed for the seismic design of critical piping and equipment supported in nuclear power plant structures [14, 63a].

These procedures account for equipment-structure interaction and provide a rational basis for design. These procedures are summarized in the following

paragraphs

.

If tfie mass of the equipment is significantly less than the mass of the

floor on which the equipment is supported (as often the case), then the

equipment and structure can be uncoupled. That is, the equipment and structure can be analyzed as separate items. An analysis of the supporting structure can be made by either the time-history, modal superposition method or the direct integration method to determine the time histories of motion at

the various floor levels of the structure. These time histories can then be

used to compute s i ngl e - degree-of -freedom damped response spectra for each of

the floor levels. Examples of the mathematical model for a reactor containment and resulting response spectra are shown in Figure 12 [63a]. Once the

response spectra have been determined in this manner, it is then possible to

design the equipment to resist the appropriate seismic forces. Several

approaches to accomplish this can be used: approximate analyses, detailed

analyses, and testing.

Approximate analyses can be performed as follows. Regardless of the

natural frequencies of the equipment, the equipment can be designed for a static coefficient equal to some factor, say 1.5, times the peak of the floor

385
Spectra from

Time Histories

of Floor Accelerations

Time History

Dynamic Analysis

of Building

Smooth Spectrum

Postulated for

Site

Time Histories

of Floor Accelerations

Time History to "Match" Spectrum

P777, Model

To

FIGURE 12. SEISMIC ANALYSIS STEPS

386
response spectrum. This will ensure a safe design. This approach has the

advantage that it is not necessary to compute any equipment frequencies, but

the disadvantage is that the seismic loads may be quite high in some cases.

This of course is not a significant disadvantage if the equipment has high

inherent strength.

If it is not elected to perform approximate analyses, then the engineer

must resort to detailed dynamic analysis procedures. The first step in such

an analysis is to compute the natural frequencies (and mode shapes) of the

equipment. If the equipment is quite rigid (with a fundamental natural frequency higher than, say, 25 to 30 cps), it can be designed for a static

coefficient equal to the peak acceleration of its supports. If the equipment

is flexible, then the dynamic response can be obtained by the response spectrum, modal superposition method.

If the mass of the flexible equipment is not significantly less than the mass of the floor on which it is mounted, then the equipment and structure

must be analysed together as a coupled system. This approach is usually

followed in the analysis of the nuclear steam supply system and supporting

structure in a nuclear plant, but most likely would not be required for

equipment in conventional building structures.

In lieu of the aforementioned analytical methods of ensuring seismic

adequacy, testing of equipment may be substituted. The basic requirement of

any testing program is that the testing machine should be capable of simulating the levels of response indicated by the response spectra applicable at

the points of support of the item to be tested.

A number of special precautions warranted in the seismic design of

equipment are listed in Table 2 [63]. The application of the above approaches

described in this section, including approximate analyses, detailed analyses,

or testing, provide a rational basis for ensuring the seismic adequacy of

equipment supported in buildings.

Table 2 SPECIAL PRECAUTIONS AND RECOMMENDATIONS

1. Design Equipment Supports for Toughness and Ductility. Support materials

(such as mild steel) and support arrangements should be designed to provide tough performance. Brackets, anchors, etc., should be ductile so that they can bend and not break. They can thus continue to carry load

and absorb energy.

In special situations, non-ductile materials such as insulating porcelain

must be used to support equipment, especially electrical substation

equipment. Because of the lack of ductility these materials exhibit,

the suggested coefficients may not be large enough. Dynamic analysis

can be employed to determine forces and displacements, or design loads

can be increased. Some means of attenuating the seismic motion may also

be needed to protect the equipment.

2. Connections are Important. Many engineers consider it good seismic

design practice to balance member strength with connections as strong

as the member itself or to provide connections that bend or give rather

than break.

3. Consider the Interaction of Piping and Structure. Pipes attached to

relatively massive, rigid appurtenances and structures can be broken

or damaged by differential displacements due to foundation rocking or

387
differential foundation settlements or displacements caused by vibration-induced soil consolidation.

Distress to piping and hangers can also result where a stiff piping

system tends to resist the sideway of the more flexible structure to

which it is attached. The stiff piping may tend to act as a lateral

force resisting element of the flexible structure - something piping

is generally not designed to do,

4. Avoid Resonance Problems - Mount Equipment Rigidly. Equipment flexibly

mounted in buildings or structures can have its response to earthquake

motion considerably amplified. When the period of vibration of the

equipment system is approximately the same as one of the predominant

periods of vibration of the building, amplification of 10 times the

building response or even greater can result. Possible resonance

problems can be avoided by securing or bracing equipment rigidly.

5. Consider the Effect of Interstory Building Displacements. Equipment

secured between two floors of a building may be forced to accept large

interstory displacements under earthquake motion. This situation can result in distress to equipment and supports, particularly when the

equipment system is rigid in comparison to the building and tends to

act as a lateral force resisting element of the building. Care should

be taken by the designer to assure that the equipment supporting system

can accommodate realistic interstory displacements, which may be on the

order of two to four times greater than those calculated from code

sei smi c forces

.

6. An Experienced Engineer Should be Responsible for Seismic Design of

Equipment. Seismic design of equipment supports should be accomplished

by an engineer familiar with structural analysis techniques and experienced in structural design in seismic areas.

VI. Design Procedures

Design procedures have been covered in some detail in the preceding

text (Sections II and III). Present philosophy of building codes in the

United States has been discussed by Pinkham. However, it should be noted that

the trend of future codes appears to be in direction of requiring, in design,

more consideration of all aspects of the earthquake engineering problem:

probable earthquake motions at a site or in a given area, effects of geologic

structures underlying and surrounding a site, effects of site foundation

materials, dynamic characteristics of structures, effects of horizontal and

vertical motions, and effects of earthquake-induced motions on nonstructural

elements, equipment, and systems. Indications of this trend are evident in

the proposed new earthquake code for the City of Los Angeles [19] and in the

Position Paper recently prepared by the SEAOC Seismology Committee [67].

Future codes must consider the aspects listed above, but they should be

codified and written in a manner that can be understood and utilized by the

practicing design professionals and the building officials who will be

charged with administering them. The dynamic characteristics of earthquake

motions and of the soi 1 -structure systems must be taken into account in building designs. This does not necessarily mean that all buildings need to be

designed by dynamic analysis methods. It does mean, however, that recognition

must be given to all of the factors involved.

It is hoped that as more is learned about computer analyses and simulation of building response, standardized procedures may be developed. That is

388
structures that are relatively uniform and symmetrical in arrangement

could be analysed using simpler models than for those more irregular in shape.

Some attempts have been made to categorize buildings by structural type - moment resisting ductile space frame, moment resisting space frames, moment

resisting - partial shear element buildings, and shear wall structures.

Present code provisions tend to recognize the decrease in response of longer

period high rise buildings, however, for low rise structures the possibly

higher response of flexible framed structures as compared to stiffer sheartype buildings is not recognized in the present codes.

It would seem possible that soil effects for a given city or region could

be investigated, correlated, and published. The effects of each type of site,

considering construction type and fundamental period of structure, could be

determined and codified. Lateral and vertical seismic force factors could

then be assigned for typical small buildings. A similar approach could be

followed for medium-rise buildings.

ACKNOWLEDGEMENT

A number of people contributed to the development of this paper.

Mr. Christian Meyer of A. C. Martin assisted in the writing of Section II of the paper. The Seismology Committee of the Structural Engineers

Association of California reviewed the text; in particular Messrs. H. S. Kellam, J. F. Meehan, and D. Strand provided a number of helpful comments

and suggestions. The efforts of the Engineering Support Services group and

typists at John A. Blume & Associates, Engineers made it possible to meet

the tight schedule for preparing the paper. To all of the above, the authors

extend their heartfelt thanks.

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394
SURVEY AND EVALUATION OF EXISTING BUILDINGS

by

Frank E. McClure*

I . Introduction

Objecti yes

The objective of this paper is to present the state-of-the-art concerning

the "Survey and Evaluation of Existing Buildings."

This review includes:

1. Evaluation of practices in current use and identification of the

best current practices.

2. Identification of opportunities and priorities for developing

improved practices from documented research findings.

3. Recommendations for high priority research activities to fill gaps

in knowledge.

Background

Buildings constructed with earlier building practices require special

evaluation of hazards from earthquakes, extreme winds, and similar loadings.

In emergency and post emergency situations, evaluations of deteriorated and

damaged buildings are particularly needed to determine whether they are safe

for immediate use and whether repairs are economically feasible. Screening

and detailed evaluation procedures both must be used. Relatively simple

screening procedures may be capable of identifying many buildings as either

clearly adequate or clearly unduly hazardous. Since strengthening or con- demnation is very costly, procedures and criteria for evaluation of questionable buildings may be more refined than those used in conventional design.

The post-disaster period is considered to be the period of emergency

relief, recovery and rehabilitation programs. The pre-disaster period is

the period after these programs have been completed or the period before the

disasters. The pre-disaster period is longer than the post-disaster period.

It exists in those areas of the United States which have a potential for

natural disasters even if these disasters have not occurred in historic time.

The procedures and criteria for the post-disaster and pre-disaster

periods differ because public policies change with the immediacy of the

natural disasters.

Work Statement and Scope

The scope of work for this paper includes the following:

1. Review published and unpublished work of others in the survey and

evaluation of existing buildings.

*Partner, McClure & Messinger, Consulting Structural Engineers, Oakland,

Cal i forni a

.

395
2. Review applicable portions of reports prepared by the author in his

pre-di saster and pos t- di s as te r studies.

3. Contact engineers, government and military representatives, contractors and others with first-hand experience to compile and review

their recommendations on damage surveys and demolition or recon- struction work following disasters caused by earthquakes, extreme

winds, and similar loadings. The sampling, though concentrated in

the Pacific Coast "earthquake country," will endeavor to include in

its scope representative findings in areas of the nation subjected

to other destructive forces.

4. Prepare a paper based on the experience of others and the author's

previous work leading to recommendations for damage survey and

repair procedures, plus conclusions regarding the enforcement of

governmental ordinances.

The main subject topics presented in this paper are:

1. Pos t- di s as te r procedures for mobilizing inspection teams.

2. Procedures for evaluation of the condition of structures.

3. Procedures for repair and the degree of rehabilitation attainable.

4. Rehabilitation ordinances and their effectiveness.

5. Pre-disaster inspection of existing structures to assess their

potential hazard risk and pre-disaster ordinances.

6. Recommendations for Federally sponsored studies on legal controls

of rehabi 1 i tati on

.

7. Compilation of a bibliography of available papers and articles on building damage surveys and repair.

Post-disaster procedures and ordinances are discussed first because

their pre-disaster counterparts usually develop from the post-disaster experiences. Proper implementation of pre-disaster procedures will mitigate the

need for some of the post-disaster abatement programs.

II. Pos t- Di sas te r Procedures--Earthquakes

Introducti on Recent natural disasters caused by earthquakes, hurricanes, tornadoes

and extreme winds have tested procedures for the mobilization of inspection

teams to survey and evaluate existing buildings. The work of surveying and

evaluating is usually under the jurisdiction of the local building departments. These inspections are made in two phases. Phase 1 inspections are made during the emergency period immediately following such disasters. Phase

2 inspections after the immediate emergency is past provide the opportunity

for more complete evaluation of the condition of the buildings preparatory

to actual design of repair and rehabilitation work and the implementation of

actual construction.

This section of the paper discusses the experiences of various building

departments during the emergency period. Other portions present their procedures for evaluation of the condition of the buildings during the recovery

peri od

.

396
The recent 1971 San Fernando earthquake is a logical starting point in

such a review of mobilization procedures. The experiences of building departments in the 1969 Santa Rosa, 1964 Alaska, 1954 Eureka, ]95Z Kern County and

1933 Long Beach Earthquakes provide additional background in reviewing the

history of mobilization and evaluation plans.

A building department's approach to damage inspection and rehabilitation

is usually based on a four-phase program: 1) Immediate survey by inspectors

to determine which buildings are damaged and to post those buildings which

are unsafe for occupancy, 2) Resurvey by building department engineers or private consulting structural engineers hired by the local jurisdiction to

re-evaluate the doubtful buildings and verify those buildings that are posted

as unsafe, 3) Withholding occupancy until the owner's private consulting

engineer renders a report that the building has been restored to its original

vertical and lateral load resistance, or to other criteria developed immediately after the disaster, and 4) Plan-checking and field inspection of the

rehabilitation work.

Immediately after the disaster, in the emergency period, the building

department's primary mission is to obtain an overall evaluation of the damage

to aid the local elected governing bodies In making decisions as to whether

or not to request aid under state and Federal Disaster programs. Their next

most immediate mission is to survey buildings to determine whether they are safe or unsafe for occupancy. With the advent of state and Federal Disaster

programs, building departments are requested to provide statistical data in

terms of dollars of damage and the number of buildings and people who have

been affected by the disaster. Unfortunately, most building departments have

not been adequately prepared to provide the various types and detailed

statistical information requested by Federal, state and local agencies. This

paper will provide recommendations to overcome this deficiency.

"Unsafe" buildings are often occupied after earthquakes, because additional hardship to disaster areas due to unemployment and cessation of

business activity must be minimized. This places an additional burden on the building departments to permit immediate occupancy of shored-up "unsafe"

buildings while permanent repairs are worked out.

San Fernando Earthquake

The mobilization of inspection teams for the large jurisdictions, such

as the City of Los Angeles and the County of Los Angeles, and the smaller

cities were different. The larger jurisdictions usually had pre-disaster

emergency plans, which were set forth in Disaster Orders, that were developed

from their atomic bomb attack response programs. Some of these disaster

plans were more than just "paper plans" and represented actual training and

disaster drill exercises, in addition to actual disaster plan implementation

under fire, floods, and other similar disasters. Few of the smaller cities

had such wel 1 -prepared disaster plans. Some had to rely on State Mutual Aid

Disaster Programs or plans developed immediately after the earthquake.

The experience of the City of Los Angeles Department of Building and

Safety in this earthquake represents the state-of-the-art for the mobilization of inspection forces by a large city with a pre-disaster plan in the

event of a moderate earthquake. Immediately after the initial shock, the

Department was faced with the realization that there had been an earthquake,

but they did not know where the damage was concentrated or the extent of the

damage. The Department divided the City into 100 districts and sent inspectors into each district to survey the conditions and report back within three

hours. The inspectors were given copies of the "Earthquake Damage Survey

Guide [8] prepared by the Earthquake Engineering Research Institute in 1964,

which the Department had on hand in accordance with their pre-earthquake

disaster plan. Inspectors sent to the heavily damaged areas were able to

397
enter the areas early in the morning but could neither telephone nor leave

the areas because of the breakdown in telephone communication (they had no emergency radio communication capability) and because, in many cases, the areas had been barricaded by police. It was not until 12 hours after the

earthquake that the Department had an appreciation of the magnitude of the

disaster and knew where the damage was the heaviest. Helicopters were used

to a limited extent to attempt to identify areas of heavy damage. It is

recommended that in future disasters more use be made of helicopters in

conjunction with a training program involving the Department's engineers and

inspections, so their use could be maximized. Aerial photographs taken at

low altitudes are useful in documenting the damage.

The second day the inspectors were sent to specific addresses in disaster

areas based on requests for inspection by the owners. By the fourth day there

were 2,600 unanswered requests for inspections and it became apparent that

simply answering called-in requests for inspections would not be adequate to

insure public safety. A house by house, block by block survey was commenced

in the heavily damaged areas.

The inspectors were given emergency building inspection forms and unsafe

building posters and were told that they were to determine if buildings could

be safely occupied or should be vacated and posted "unsafe." They were to

make this decision based on their evaluation of the extent and type of damage

and the possibility of after shocks which could further damage the buildings.

There was no time for a complete write-up of the damage for each damaged

building. Within two weeks, all buildings which had been ordered vacated by

the initial inspections were re-inspected by the Department Structural

Engineers to determine the correctness of the initial decision to vacate

unsafe buildings. Many owners who had never experienced an earthquake

refused to return to their homes until an inspector assured them it was safe

to reoccupy.

The biggest challenge, which was underestimated in the pre-disaster

plans, was trying to furnish information and statistics requested by as many

as 20 outside agencies. The emergency building inspection forms were not

designed to collect information concerning the dollar losses. More than

25,500 buildings were inspected, and since the damage inspection forms were

not coded for computer application, all statistics had to be compiled manually. One serious problem developed when the same owner would call back

several times requesting an inspection. There were no adequate provisions

for keeping an up-to-date record of whether or not the inspection had been

made. This resulted in duplicate inspections of many buildings.

The work accomplished was possible only because the telephone service

did not break down except in the heavily damaged areas. The Department's

efficiency would have been reduced to 10 per cent if the earthquake had been

of a greater magnitude and had the telephone service failed. It is recom- mended that an emergency communication system, probably radio, be established

for use by the Department. Without Federal aid the possibility that this

will be done is very remote, due to lack of local funds.

Because of a four-day weekend, the Department was able to make all the

critical inspections without calling in outside private consulting engineers.

At one point, however, the Department was seriously considering, under the

State Mutual Aid program, a request to the cities of San Diego and San

Francisco for emergency help. Had the earthquake been of greater magnitude,

and had such outside help been needed, some plans for the use of inspectors

who were not familiar with the local areas should have been prepared in

advance and made ready for use. In a great disaster, the local building

departments cannot expect too much help from local structural engineers and

architects who will be committed to work requested by their clients.

The City of Los Angeles, Department of Building and Safety, has recently

398
made a critical review of their response to the San Fernando earthquake and

has made revisions in their 1967 Disaster Orders and their Emergency Inspection Forms. The new forms are formulated for computer analysis. A detailed

study of the experience of this Department and the basis for the revised

Disaster Orders and their Emergency Forms is strongly recommended.

The County of Los Angeles' experience was similar to that of Los Angeles

City. The Building and Safety Division had a standard procedure for inspection and reporting of damage as a result of fire, flood, earthquake or other

disaster. The Division had on hand standard report forms for reporting of

damage, including provisions for reporting the estimated dollar value of the

damage. The information on these report forms was combined with data obtained

during the resurveys on computer data cards which could be read directly by

the computer without punching another set of data cards. The computer program

was flexible so that the information on a particular building could be easily

updated, depending on the re- i nspecti ons and changed status of demolition or repairs. It provided for the initial inspection as well as re- i ns pect i on s and

the close-out inspection. The computer capability provided for calculation of

dollar damage statistics, as well as daily monitoring of the status of each

building and the status of the total rehabilitation program. This provided

the information which was very helpful for the County Supervisors and others

in decision-making positions.

After considerable experience in the preparation and the review of

other damage survey forms used in previous earthquakes, the author was most

impressed with the computer damage survey data card developed by Los Angeles

County. The success of their computer capability in providing immediate

information concerning the overall damage as well as constant updating of

the data strongly recommends further study of the application of computers

in disaster surveys

.

The experience of the building departments in smaller cities in the Los

Angeles area was typical of similar cities in previous earthquakes. The

small building departments do not have sufficient staff to make the necessary

inspections and relied on assistance from the State Division of Housing,

State Office of Architecture and Construction, and the State Office of Emergency Services. They reported the lack of adequate communications, the lack

of unsafe building posters and inspection forms, the need for improved

methods of presenting damage statistics to governmental agencies and private

insurance and lending agencies, and the need for improved procedures for

requiring building demolition. A serious need was to have an up-to-date predisaster written plan with provisions to cover specific disaster situations.

More detailed discussion is presented in References 1 and 2 concerning

the pre-disaster plans, response, problems and recommendations of the building departments in the Los Angeles area as a result of the San Fernando

earthquake. References 3, 4, 5, 6 and 7 review the experiences of building

departments following previous earthquakes.

III. Post- Pi sas te r Procedures - Other Natural Disasters

Introducti on

For the purpose of this paper, "other natural disasters" are hurricanes,

tornadoes and extreme winds as well as explosions and accidents which produce

effects similar to earthquakes. Although the main emphasis in this paper is on earthquake disasters, the procedures and recommendations for earthquake

disasters are applicable to the other natural disasters. The author's professional practice has been almost exclusively in the earthquake field;

however, he experienced two typhoons while on Okinawa in 1945. Review of the

reference material for these other natural disasters points up the similarity

399
of the response of buildings during typhoons and their response to other

meteorological ly-caused disasters.

Tornadoes, with their whirling winds, are the most violent weather

phenomena known to man. During the past 50 years U.S. tornadoes and severe windstorms have killed amost 18,500 persons, while hurricanes and floods have

killed approximately one-half that number during the same time period. The

hurricane brings devastation by wind, flood-producing rain and the storm

surge which is the most lethal. Considering only the damage caused by the

wind effects of such disasters, the losses to property are approximately

equally divided among ordinary gales, tropical hurricanes, and tornadoes [24].

Procedures

The areal extent of these other natural disasters and their dollar

losses are usually so great that detailed post-disaster building investigations ordinarily are not made. The inspections are often undertaken by

Federal agencies and their consultants who have responsibilities for the

prediction and warning of these disasters. These agencies, obviously, have

the best understanding of the mechanisms which cause these disasters, but

they seldom have the in-house capability to assess the structural performance

of the buildings except in general terms. The literature is very limited in

detailed damage statistics by type and quality of construction. Few studies

similar to that of K. C. Mehta and A. J. Sanger in 1971 [22] have been made.

Host studies for the larger disasters, such as the 1969 Hurricane Camille,

[21] have been made by airplane. The basis and detail for the dollar loss

statistics customarily are not given in the reports of such disasters. Those

Federal agencies which have a responsibility for the review of the performance of materials and the review of the adequacy of building code provisions

have not been involved in post-disaster surveys to the extent that they should

for adequate disaster mitigation through improved building practices.

Damage Evaluation

Although the author has personally observed only the damage caused by

typhoons, a review of the descriptions and photographs of the damage caused

by other disasters leads to the conclusion that in many respects the damage

caused by earthquakes and the damage caused by these other disasters is

quite similar. Unreinforced masonry walls collapse, while reinforced masonry

wall construction performs quite well. Wood frame dwellings which are not

bolted to the foundations are often destroyed while dwellings with adequate

anchorage to the foundations perform well. The degree of damage depends more

on the quality of construction than on the magnitude of the tornado and

hurricane wind forces [21, 22, 23]. This is not to say that the other disasters subject the buildings to the same type of loadings as do earthquakes,

but buildings that perform well in these other disasters have many of the

characteristics that are important in earthquake-resistant construction.

These other disasters subject the buildings to negative and positive pressures

and uplift forces due to negative air pressure gradients. However, a building

which is tied together so that it functions as a unit and has a rational system for the resistance of lateral forces, using materials of adequate strength

and ductility, will perform well in other wind-caused disasters as well as in

earthquakes.

Disaster Mitigation

The present state-of-the-art for disaster mitigation from other disasters

is largely based on adequate public response to prediction and warning programs. These programs have been very effective in reducing the loss of life.

Until weather modification programs can appreciably reduce the intensity of

400
these other disasters, however, buildings must continue to be subjected to

the forces from these disasters. Unfortunately, without complete disaster

damage statistics by type and quality of construction, it is generally

assumed that the forces from tornadoes, hurricanes, and violent windstorms

are so great that few buildings can be economically designed to resist these

forces. This is not true and the few structural surveys have been made support the conclusion that improved building practices can mitigate the effects

of these other disasters [21, 22, 23].

The recommendations for improved building procedures given in References

21 through 27 to make buildings more wind-resistant are basically the same recommendations for making buildings earthquake-resistant. Engineers experienced in earthquake-resistant design will recognize the similarity in these

recommendations, because these recommendations are included in their earthquake-resistant designs almost as a matter of course.

The main impediments to the implementation of these recommendations are inadequate building codes, inadequate enforcement of these codes, and lack of

understanding by the local authorities, local architects, engineers and building contractors of the benefits to be derived from the application of the

simple principles of lateral force design.

It is recommended that an appropriate Federal agency be given the

responsibility to encourage local application of the present state-of-theknowledge for disaster mitigation through adequately enforced and improved

building codes. Federal agencies which have statutory responsibilities for

the funding of Federally-supported building projects and the review of the

adequacy of construction materials must play a more active role in the investigation of other disasters as well as the testing of building components to

resist the forces from these disasters.

IV. Evaluation Procedures

Introducti on During the emergency period, the main question is whether the building

is safe or unsafe for occupancy. This decision is usually based on a cursory

inspection of the building and an on-the-site judgment whether the building

is adequate for vertical loads and lateral forces which might occur prior to

the start of repair work. After the emergency period, when there is time for

a more complete evaluation of the damage, the evaluation procedures vary

according to: 1) The type of construction of the building, 2) The degree and

the causes of damage, and 3) The building department's criteria for the degree

of rehabilitation required.

Except for the "Earthquake Damage Survey Guide," [8] no publication is

generally available covering how to evaluate the damage to buildings caused by

lateral forces--wind and earthquakes. Inspectors, who do not have formal

training in structural engineering, must rely on their observations of the

observable evidence of the strains in the building and make judgments as to

the severity of the damage based on their practical knowledge of good con- struction. Earthquake damage, short of collapse, is not spectacular and it

takes an experienced investigator who has inspected previous earthquake

damage to detect the subtle indications of such damage. Structural Engineers,

who have experience in earthquake-resistant design and who have either made

previous earthquake damage inspections or who have read the reports of previous earthquake damage, know where to look for earthquake damage and how to

evaluate this damage.

The state-of-the-art of detailed evaluation of damaged buildings is not

fully developed for adequate protection of the public because the relative

401
infrequency of major disasters prevents effective updating of the art and

because of the limited number of experienced professionals who have made such

i n ves t i gat i ons .

Procedu res Before any detailed evaluation of the damage to individual buildings

(except hospitals, emergency relief centers and other buildings with high

socio-economic importance) is started by governmental agencies, an inventory

of all damaged buildings based on cursory inspections must be compiled. The

damaged buildings should be reported on simple, one-page forms or on computer

data cards similar to those used by the County of Los Angeles in the San

Fernando earthquake. A more detailed report form is required for the secondphase, detailed evaluation of the damage. Although private consulting engineers working directly for building owners will probably start prior to the

completion of the inventory of all damaged buildings by governmental agencies,

it is recommended that these engineers use the same standard, second-phase

report forms provided by these agencies. This procedure will result in a consistent method of reporting the damage by the private engineers and will

aid in expediting the approval by the building departments of plans for the

repair. This is not to preclude the private engineers from preparing their

own detailed evaluation reports in formats reflecting their individual

professional practices, but if the standard, second-phase reports were

appended, this would lead to a consistent approach and aid the building

departments in their evaluation of the private engineers' reports.

The 1964 EERI "Earthquake Damage Survey Guide" [8] and the 1970 Report

by the Structural Engineers Association of Northern California, "Emergency

Earthquake Damage Survey Procedures and Guidelines for Earthquake Damage

Repair" [9] outline detailed procedures for more complete evaluation of the

condition of structures. The author contributed to both these reports.

Based on a knowledge of the background and thinking that went into these

reports, it is recommended that these reports be reviewed in light of the

experience in the San Fernando earthquake and that a combined report be

prepared. The new report should be broadened to cover all disasters; it

should include mobilization plans coordinated with State and Federal Mutual

Aid and Disaster plans; it should include the two-phase approach: Phase 1,

the emergency phase, to determine the overall extent of damage and the

unsafe buildings with more information on the dol 1 a r- damage statistics, and

Phase 2, the post-emergency phase, to establish procedures for more detailed

evaluation of building damage. The two-phase report forms should be developed

to provide for both computer and non-computer data analysis similar to the

Los Angeles County approach, and the report should be revised to include an appendix which could be quickly read to aid those not familiar with disaster

building damage evaluation on what to look for and how to fill out the forms.

The interest and concern of even the most devoted professionals, except

those directly involved on a day by day basis in disaster programs, tend to

diminish between disasters. Although the Structural Engineers Association

of Southern California continues to have a Disaster Study and Report Committee, similar committees have either been abandoned or have not been formed in

the other three SEAOC Associations. There is a serious need to establish a cadre of concerned professionals in all professional building construction

associations who will annually meet and review their response programs to

future disasters .

The space limitations of this paper do not allow a complete discussion

of the procedures for full evaluation of the condition of structures after

the emergency period. The state-of-the-art of such evaluations is represented

by References 8 and 9.

402
Introduction

V. Repair Procedures

The terms "repair," "rehabilitation," and "reconstruction" are often

used interchangeably. According to Webster, "repair" is to put back in good

condition after damage; "rehabilitation" is to restore or to put back in good

condition; and "reconstruction" is to construct again, rebuild, or make over.

For the purposes of this paper and for brevity, the word "repair" is used for

all three terms. Strictly speaking from a construction view-point, to

"repair" or to "rehabilitate" means to put back or restore what was in the

original construction. To "reconstruct" is taken to mean to construct again

and to add materials which were not in the original construction.

Little has been written in the American literature and very little

emphasis has been given to the engineering criteria for the repair of damaged

buildings after disasters. No aspect of structural engineering tests the

ingenuity of the Structural Engineer more than does the preparation of a design for the shoring, temporary bracing, and permanent strengthening of an already damaged structure. When a disaster strikes a community, after the

emergency period has passed, the primary concern of the community is to

restore its economy to its pre-disaster condition. Buildings must be repaired and placed in service as soon as possible and there is little time or concern for the niceties of design or the general philosophy of safety. In many

instances, buildings have been repaired using the "patch and paint" approach

to cover up the evidence of the damage which precludes the proper investigation of the extent of damage [10].

There are two broad categories of damage: 1) Non-structural and 2)

Structural damage. The non-structural damage--the cracked plaster, shattered

partitions, dropped ceiling, fallen light fixtures, and displaced mechanical

and electrical equip me nt--is usually repaired as rapidly as possible. In the

general confusion after the disaster, there is a loss of control by the

building departments and minor repairs are completed without governmental

control and inspection. Usually little consideration is given to the remaining structure if it was not seriously damaged [10].

The structural damage might not be evident without the removal of

ceilings, plaster fire-protection, and other finishes which can hide the

damaged structural elements. It requires a very conscientious structural

engineer to require the removal of finish materials so he can make very

careful examination of the evidences of damage to the structure. Liberal

sampling by means of test cores to establish the strength of concrete or masonry and test specimens to establish the strength of reinforcing is

recommended particularly when the engineer did not design the original

building or drawings and specifications are no longer available. Without

the construction drawings, the engineer must rely on his knowledge of con- struction practices at the time of construction as well as his engineering

evaluation of what the construction that he can see actually represents [9].

Extent of Repair and Reconstruction

One of the most difficult decisions to be made is to what level the

building should be repaired. If the building collapsed, there is no problem.

If the building is seriously damaged, however, and there are no pre-disaster

building code requirements concerning the degree of rehabilitation, what

level should the engineer use in his design of the repair? Five possible

approaches are:

1. Repair to the extent necessary to provide for stability under

vertical loading only.

403
2. Restore the structure to its original strength. This is usually

the highest level attempted by building departments without predisaster criteria to the contrary. The argument for this level of

strength is that it brings the building up to the legal level of

safety under which the building was originally designed. It overlooks the fact that the building was proved weak in a disaster

and will probably suffer similar damage in another disaster.

3. Repair the major structural elements to their original level of

strength and reinforce the failed members to a higher degree of

strength

.

4. Reconstruct to the level of strength and safety set by the latest

current pre-disaster standards of design and construction being

enforced by the building department for new buildings. The

structure will probably be stronger, but the design of the recon- struction will not include the new knowledge that may have been

developed after the disaster.

5. Reconstruct to the highest level based on the new knowledge or revised pos t- di s as te r standards of design and construction. This

- ' could result in some delays due to the time to codify the new knowledge while the owner is pressuring the engineer to finish his

design and get the building back in operation as soon as possible.

This procedure was used by the Venezuelan building authorities by means of provisional standards which were developed soon after the

earthquake to guide the engineers responsible for the repairs [10].

Henry Degenkolb was one of the first to present these concepts in the

report, "The Venezuela Earthquake, July 29, 1967" [10]. It must be recognized

that the owner possibly faces very large repair costs to bring the damaged

building up to a safe lateral and vertical force requirement. The 1970 Uniform Building Code has specific provisions concerning the degree of repair

which vary with the percentage of the dollar value of the repair to the value

of the building. It also has very specific provisions in Volume IV, "Dangerous Buildings," for the abatement or reconstruction of buildings damaged by

disasters. After the 1952 Bakersfield earthquake and the 1969 Santa Rosa

earthquake, similar pre-earthquake provisions were not enforced because of

the potential adverse economic impact on those cities and modifications of

the provisions were voted by the local city councils. (See section "PostDisaster Rehabilitation Ordinances.")

When California public schools are reconstructed under the provisions of

the Field Act, the reconstructed schools must be designed and reconstructed

to the level of safety provided by the current provisions of the Field Act

for new schools. This has been the law since 1933 when the Field Act was first enacted. Hundreds of pre-Field Act schools have been demolished and

replaced, however, because - in addition to questions about the schools'

adequacy educationally - the cost of reconstruction has been so high compared

with the cost of new construction that it was deemed prudent to replace rather

than reconstruct.

Methods of Repai r Methods of repair will necessarily vary with the type of construction,

the type of structural element being repaired and the degree of damage. In wood or steel structures, where adequate shoring is possible, removal and

total replacement of damaged members is a relatively simple procedure. Repair

of cracked or spalled concrete members is more difficult, particularly if the

reinforcing steel has elongated. Replacement of cas t- i n-pl ace or pre-cast

elements usually involves the breaking out of the members and the repouring

and repacking of the same member. According to the 1970 Uniform Building

404
Code, unreinforced masonry walls are not permitted to be used if they resist

seismic forces. Therefore, they must be strengthened by other materials such

as reinforced masonry, gunite, or poured concrete.

Strengthening of a structure by the addition of concrete shear v;alls,

unless properly located, can introduce torsion problems where none existed

and the building could have a lower total strength than it had before recon- struction. Addition of new lateral and/or vertical 1 oad- res i sti ng frames or elements can lead to problems of overloading of existing footings due to

additional dead loads and overturning forces from lateral loads. If no soils

information is available, soil investigations might be required to review the

adequacy of existing and new foundation systems [9].

Degree of Rehabilitation Attainable

The most important influence on the degree of rehabilitation attainable

is the ability to repair or reconstruct the building so that the final structure has a rational system for the resistance of the vertical and lateral

loads using materials of adequate strength and ductility. A high degree of

repair can be achieved in wood-frame and light steel-frame structures. The

future behavior of such structures is influenced by whether or not the

structural steel frame members and connections have gone into yield and have

lost a portion of their ductility. If these members and connections have

yielded and some have not been identified and replaced, the degree of rehabilitation is appreciably reduced [10].

Relatively highly shattered concrete members can be almost completely

repaired by the introduction of epoxy under pressure into the cracks, by

breaking out shattered concrete and replacement of this concrete with drypack

epoxy mortar, or the bonding of newly-poured sections to existing concrete

by troweled-on epoxy. Repair of concrete members is greatly influenced,

however, by the adequacy of the workmanship and the proper use of these

special epoxy materials. It is estimated that the pressure epoxy-grouted

repair members can attain a strength of 80 per cent of the original members,

but this requires continuous inspection including test coring to check the

penetration of the epoxy [9].

Gunite (pneumatically applied concrete) is one of the traditional

materials used in the repair of damaged concrete members or the introduction

of new shear walls. One of the disadvantages of this type of repair is the

extreme difficulty of proper placement in areas of congested reinforcing.

If the initial preparation and application is properly performed, it is

reasonable to expect that members so repaired or added will perform adequately [9].

The repair of damaged buildings is a new, relatively untried art that

has generally been neglected by engineers. The need for research in this

field is vitally important to the economy and public safety in areas that

will experience disasters [10]. The research should include: 1) Testing of

members and connections which have been stressed to yield and above to

determine the degree of degradation of strength and ductility when subject

to repeated above-yield loadings, 2) Testing of repaired members and connections which have first been stressed to yield and/or failure to determine the

degree of restoration obtained in terms of strength and ductility, and 3)

Testing of various methods of repair for various construction materials to

compare the relative degree of restoration achieved. Examples are the testing

of various methods of application of epoxy to repair concrete members, the

testing of welds to repair cracked or insufficient welding, and the testing

of insert connections.

405
VI. Pos t- Di sas ter Rehabilitation Ordinances

Introducti on

This section discusses the various ordinances and their enforcement

which were in effect prior to specific disasters and ordinances which were

passed soon after these disasters, compared with ordinances which were passed

after the recovery and redevelopment period. Disasters in the first category

of ordinances are the 1933 Long Beach, 1952 Kern County, 1969 Santa Rosa, and

1971 San Fernando earthquakes. Ordinances in the second category were usually

passed years after the disasters that focused attention on the need for such

ordinances. These latter ordinances and their enforcement are discussed

under "Pre-Di saster Procedures and Ordinances."

1933 Long Beach

The California State Legislature was in session on March 10, 1933, when

the Long Beach earthquake caused serious damage to many public schools in

Southern California. In order to protect the children and their teachers

against possible death and injury from unsafe public schools, the Legislature

enacted the Field Act [28], effective April 10, 1933, one month after the

earthquake. The Field Act vests with the State of California, Office of

Architecture and Construction, the authority and responsibility to pass upon

plans, specifications, and construction of any new public school building

constructed since April 10, 1933, regardless of cost, or if the estimated

cost of reconstruction, alteration of, or addition to any public school

building exceeds $10,000. The Field Act was not and still is not retroactive . It does not specifically apply to pre-1933 public schools unless they are altered, repaired, or reconstructed. The Field Act requires that any con- struction must be designed and constructed to provide a level of safety

equivalent to new construction. The provisions of the Field Act have been

strictly enforced and any violation of the Act is punishable as a felony.

School buildings constructed and reconstructed under these provisions have

been subjected to the 1940 El Centre, 1952 Kern County, 1969 Santa Rosa, and

1971 San Fernando earthquakes, and these schools have performed very well.

1952 Kern County

The earthquakes of July 22 and August 21, 1952, destroyed or damaged

numerous buildings in the City of Bakersfield, California. The City was operating under the 1949 Uniform Building Code which required that if repairs

to a building exceed 25 per cent and were less than 50 per cent of the value

of the building within any twelve month period, the entire building would not

have to be made to comply with the requirements for a new building. If the

repairs exceeded 50 per cent of the value of the building within any twelve

month period, then the entire building would have to be made to comply with

the requirements for a new building. The repair work itself had to comply

with the requirements for the building code then enforced. If the repairs were less than 25 per cent, lesser restrictions were required [6, 7].

The City Council passed an ordinance which lessened the code requirements

for certain structural features used in the rehabilitation of earthquakedamaged buildings. This ordinance came about through pressures brought to

bear upon the City Council by owners, building contractors, engineers and

architects to bring an economical solution to rehabilitate earthquake-damaged

buildings to a minimum safe condition. The results of this ordinance brought

about the strengthening of the damaged buildings to an extent greater than

the original structural strength, but not entirely up to the pre-earthquake

code. This ordinance allowed the use of existing roof and diaphragms and

unreinforced brick shear walls which would have had to be reconstructed under

the pre -disaster code.

406
After rehabilitation was started, however, many of the buildings had

additional hidden costs besides the cost of structural repairs - in the

replacement of obsolete plumbing, electrical and mechanical features. Many owners would not have gone into the repair work had they known the complete

costs for this additional work. Instead they would have demolished the

buildings and constructed new ones.

The Building Departments in Kern County deserve a word of praise for

the corrective work that was done, compared to the previous "patch and paint"

procedures used after earthquakes in other areas. The Kern County earthquakes

showed that it was feasible to demolish badly damaged structures and repair

others effectively, providing good existing codes are not emasculated. One

of the keys to this success in Bakersfield was that a registered Civil Engi- neer was appointed as Building Official soon after the earthquake and he

insisted on enforcing the building code during the reconstruction period [6].

1969 Santa Rosa

On October 1, 1969, Santa Rosa, California, a city of 50,000 population,

was damaged by a magnitude 5.6 earthquake, which caused more than 5 million

dollars worth of damage to commercial buildings in the downtown area. The

situation in Santa Rosa was similar to Bakersfield, in that the City Council

faced the dilemma that strict enforcement of the pre-earthquake provisions

of the City Building Code - the 1967 Uniform Building Code and the 1967

Dangerous Building Code - would create further economic hardship for property

owners, displaced businesses and employees, as well as the general public.

The City Council appointed a Board of Consultants to advise the Council on interim emergency and long-term policies concerning the hazardous buildings.

On October 9 and 28, and November 4, 1969, the Council adopted ordinances

which dealt with the required minimum levels of structural safety for emergency repairs or demolition of buildings. The political policy behind each

of these ordinances was that it was in the public interest to have a balance

between the interests of the private property owners and consideration for

the public safety and welfare of the citizens of Santa Rosa. This policy was not to amend or alter the City Building Code but rather to establish an emergency policy to implement an orderly procedure to handle buildings damaged

by the earthquake [13].

The ordinances were quite detailed, and limitations on the length of this

paper do not allow discussion of all items in these ordinances. The following

are some of the more important ones:

1) No occupancy shall be allowed in a building declared to be unsafe

by the Chief Building Official and which is open to and used by the

general public until the code requirements for vertical loads are met and immediate hazards such as loose parapets and cornices are repaired or removed,

2) Emergency criteria for use of and bracing of inadequate unreinforced

or inadequately reinforced concrete or masonry were given,

3) Owners were given 44 days to comply with the emergency procedures,

4) Provisions for varying degrees of lateral bracing for the resistance

of earthquake forces were to be provided at a future date, and

5) A Structural Review Board was established to hear appeals by owners concerning the decisions of the Building Inspection Department and

to advise the City Council on what action should be taken.

These 1969 ordinances were a compromise between rather strict enforcement

of the pre-disaster City code advocated by the Board of Consultants and

407
amendments to the City code to eliminate the Dangerous Building Code provisions advocated by the property owners. Enforcement of these ordinances was influenced by the retirement, due to poor health, of the building official

who was in office at the time of the earthquake, the time interval to find

a registered structural engineer replacement, and the usual problems which a small building department faces in attempting to enforce such ordinances.

Owners were somewhat reluctant to start work on the older buildings until the

details of the degree of lateral bracing were set forth in future City Council

resol uti ons . Since 1969, the same Board of Consultants has been retained by the City

Council to develop criteria for the safety investigations of older buildings

within the City and suggestions for the reinforcement requirements for those

buildings found to be so substandard as to create an unacceptable level of

safety. These requirements are based on considerations of the economic health

of the community to develop a balance between the acceptable degree of risk

and the cost of reinforcement or demolition. These criteria are set forth in

Uegenkolb [13] and were adopted by City Resolution 9820, October 12, 1971.

Some of the important items in these documents are:

1) Buildings to be reviewed and priority for review,

2) Scope of preliminary investigation, to be conducted by representatives of the City Building Department at public expense,

3) Criteria and scope of further investigation by the property owners

and the i r engineers,

4) Building requirements for continued long-term use of structures,

5) Criteria for occupancy for a term not to exceed five years,

6) Criteria for buildings which fail to meet the requirements under

5 above, which shall be abated within one year,

7) Abatement of buildings which fail to meet the requirements for

one-year occupancy under 6 above, and

8) Other criteria for buildings which may be occupied for a period of one to five years . Time has not been sufficient since enactment of the 1971 ordinances to determine the effect. The recently appointed structural engineer building official

hired a structural engineer in mid-1972 to start work on making the preliminary reviews of the buildings. Once this has been completed, the other phases

of the work under the 1971 ordinances should progress more rapidly than did

the implementation of the requirements of the 1969 ordinances. The 1971

ordinances represent a "break-through" in ordinances for the abatement of

hazardous buildings and the City Council and its consultants are to be com- mended for developing and adopting such ordinances. Their successful implementation can have far-reaching influence on State of California legislature

for the abatement of similar buildings in all of California.

1971 San Fernando

The extent of the problems concerning the demolition and degree of

repair experienced after the 1952 Bakersfield and 1969 Santa Rosa earthquakes,

did not occur after the 1971 San Fernando earthquake except in the City of

San Fernando. The San Fernando earthquake was a moderate one, and the percentage of seriously damaged buildings was not as great as in the two previously cited earthquakes.

408
Building codes enforced in most of the Southern California cities and

counties are basically the recent editions of the Uniform Building Code which

have adequate provisions for the control of repairs to damaged buildings.

The Dangerous Building Code [12] is not always included in local building

codes, but there are similar provisions in the City of Los Angeles and the

County of Los Angeles Building Codes. It is estimated that there are more

than 15,000 old masonry-walled structures in the City of Los Angeles which

represent "clear and present danger" in the event of moderate and major earthquakes. Though abatement is clearly provided for by the pre-earthquake building codes of Los Angeles and other areas of Southern California, there is no public policy concerning the abatement of these structures that is acceptable

to the political and economic segments of the city.

Repair of damaged structures was generally executed under the planchecking and inspection procedures of existing codes. In some instances,

repair work was started immediately, before a complete investigation of the

damaged buildings could be made. This repair work covered up some of the

damage. In several buildings, due to the closer surveillance possible after

the emergency period, this repair work had to be redone. It must be remembered that this was a moderate earthquake. In the event of a major earthquake,

with thousands of seriously damaged buildings, a considerable increase in the

capability of the building departments to control the repair and abatement

work will be required.

VII. Pre-Disaster Procedures and Ordinances

Introduction

Each area of the United States is subject to potential disasters. It is

important to consider the time frame for the pre-disaster and post-disaster

periods. The post-disaster period is considered to be the period of emergency

relief, recovery and rehabilitation programs. After these programs have been

completed, the area returns to the pre-disaster period. Those areas of the

United States which have not experienced certain types of disasters in historic time may be said to be in their pre-disaster period.

The level of disaster mitigation througti identification of existing

hazardous buildings and rehabilitation or demolition of those buildings

varies with the time period under consideration. If several decades have

passed since the last disaster and there has been no active governmental program, the level of public concern is usually very low. This was the case after the earthquakes of 1906 San Francisco, 1925 Santa Barbara, 1933 Long

Beach, and to some extent 1952 Bakersfield. Examination of existing buildings

for their earthquake resistance was practically non-existent prior to the 1952

Bakersfield earthquake, even though there had been increasing concern about

the earthquake-resistant design of new buildings since 1925. It was not until

1967 that there were meaningful provisions for the investigation of public

school buildings in California built prior to the 1933 Long Beach earthquake.

A program for the investigation of California hospitals did not begin until

195G . Since the 1971 San Fernando earthquake proved the validity of the

recommendations of Steinbrugge et al , (1970), "Earthquake Hazard Reduction,"

[14] relating to the survey of non-earthquake resistant structures, activity

has been increased in the pre-disaster inspection of existing buildings.

Without an active Federal program for surveying existing buildings to evaluate

their resistance to natural disasters, the level of activity in this field of

disaster mitigation will decrease with increased public apathy. This occurred

in the late 1940s in California between the 1933 Long Beach and the 1952

Bakersfield earthquakes when legislation was introduced not only to eliminate

the investigation of existing school buildings for earthquake resistance, but

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493-610 O - 73 - 27
to eliminate the provisions of the Field Act which had proven to be one of

the best programs for the design and construction of earthquake-resistant

structures. Fortunately, it did not pass.

The following portions of this paper discuss the experience in California

of pre-disaster inspection of buildings and the results of plans for implementation of programs for the elimination of hazardous buildings. Programs

related to the public schools, universities, hospitals, and programs for

various cities are presented as well as several of the Federal programs for

the identification of hazardous buildings.

California Public Schools

The 1933 Field Act provides for inspection of school buildings when

requested by at least 10 per cent of the parents having children in the

school district or when requested by the school district. This is clearly

not an overall retroactive provision for the investigation of all public

schools. Relatively few schools were inspected under the provisions of the

Fiel d Act [28]

.

In 1939, the Garrison Act [29] provided that i_f pre-1933 public schools

were inspected and found to be unsafe for use, the school district must take

action to repair, replace or abandon the schools. Again, this was not retroactive nor did it require all schools to be investigated. After the 1952

Bakersfield earthquake, school board members expressed increasing concern

over their personal liability if they had not required an inspection of pre1933 schools and if there were to be death or injury in these schools because

of an earthquake. In 1966, the State Attorney General ruled that the school

board members were personally liable in such case.. In 1967 legislation was passed, requiring that these schools be inspected and requiring the school

board members to take definite steps with specific time deadlines to avoid

such individual liability. The reports of such investigations were required

to state whether the pre-1933 public schools were "safe" or "unsafe" based

on the sole consideration of the protection of life and prevention of personal

injury at a level of safety equivalent to that established by the Field Act,

which would be expected from one disturbance of nature of the intensity used

for design purposes in the Field Act. Few, if any, pre-1933 schools were reported "safe." No public school buildings declared "unsafe" for school

use can be used for school purposes after June 30, 1975, unless it is recon- structed to a level of safety equivalent to new school buildings designed and

constructed under the provisions of the Field Act.

In 1972, 1,593 schools reported "unsafe" were still in use. The cost of

tlieir reconstruction or replacement is estimated to be more than $600 million.

Construction bond issues for their reconstruction or replacement have failed

each time since 1970 in the larger cities such as San Francisco, Oakland, San

Diego and Los Angeles. The first Los Angeles bond issue failed even though

the election was held so soon after the 1971 San Fernando earthquake that

aftershocks were still being felt at the time of the election.

California State Universities, State Colleges and Private Schools

State colleges above the junior college level, State universities and

private schools at all levels are not covered by the provisions of the Field

Act. Junior colleges are under the jurisdiction of the Act. No pre-disaster

inspection and evaluation procedures are in effect for the private schools

similar to the 1967 Garrison Act.

State colleges and State universities are designed under the applicable

editions of the Uniform Building Code and other local building codes. The

buildings on the nine campuses of the University of California are currently

410
being investigated to evaluate their earthquake resistance using the 1970

Uniform Building Code - Title 24, State of California Building Standards - as the evaluation crite-ria. Private consulting structural engineers are performing the investigative work and are preparing estimates of the costs of rehabilitation. The need for these investigations developed out of concern of

the University of California Board of Regents for the earthquake safety of the

buildings under their jurisdiction as a result of the 1971 San Fernando earthquake. No known program of similar investigation of the other State colleges

and State universities is being undertaken.

California Hospitals

Following the 1952 Kern County earthquakes, the Department of Public

Health became concerned about the earthquake resistance of many of the old

hospitals constructed prior to the inclusion of lateral force requirements in

the local building codes. In 1955, the Department engaged a structural engineer to make cursory inspections of more than 600 such hospitals. He reported

that approximately 100 were of such construction that more detailed investigations should be made to determine their structural condition. In 1956, the

Department adopted regulations in Title 17, California Administrative Code,

which required that if the Department determined that an evaluation of the

structural condition of the hospital were necessary, then the licensee was required to submit a report prepared by a structural engineer or architect to

establish the basis for alterations to the building to eliminate or correct

the structural conditions which might be hazardous to the occupants. Letters

were written to the licensees putting them on notice of the need for such

investigations. Host notified licensees were informed by their attorneys

that they could be personally liable in the event of injury or death to

patients or staff caused by a damaging earthquake. The inspections and

reports were prepared by private consulting engineers. The progress under

this program has been good though there is no deadline for compliance with

the recommendations in the consultants' reports similar to the provisions of

the Garrison Act for public schools.

This program did not require mandatory inspection and/or repair of all

hospitals similar to pre-1933 public schools. Generally, only those judged

to be hazardous by the Department's Structural Engineer were required to

comply. Hospitals built under codes which had lateral force provisions or earlier hospitals that were not considered hazardous generally were not included. Twenty hospitals have been demolished and ninety have been reinforced

under the program.

Parapet Ordinances

More people in the United States have been killed by falling parapets,

ornamentation and unanchored masonry walls during earthquakes than from any

other earthquake cause. The City of Los Angeles recognized this problem and

in 1949 the Parapet Correction Ordinance was enacted. This ordinance was specifically aimed at those building elements which are most hazardous and

it makes it mandatory that they be corrected. The wisdom of this ordinance

was re -emph as i zed in the 1952 Kern County, 1969 Santa Rosa and 1971 San

Fernando earthquakes. The Los Angeles program was adequately funded and

administered by engineers with special public relation training. This program has been very successful, being both economically and politically

feasible [15]. Other cities in Southern California enforce similar ordi- nances which proved their merit in the 1971 San Fernando earthquake.

A parapet ordinance was passed recently in San Francisco, but has not been

implemented because of budget limitations for hiring the necessary inspectors

to carry out the program. Similar ordinances have been proposed in the

cities of Berkeley and Oakland, but these ordinances have not been adopted.

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The author attempted, with the cooperation of the Structural Engineers

Association of California, to have the City of Eureka and the County of

Humbolt pass ordinances similar to the Los Angeles Parapet ordinance,

following his inspection of the 1952 Eureka earthquake damage. To date no known action has been taken by these authorities. Certainly immediate near

term benefits would accrue if cities would pass ordinances to remove these

"clear and present" falling hazards. It is recommended that a Federal program be implemented to encourage local jurisdictions to adopt ordinances for

the removal of hazardous projections from buildings.

Long Beach

Prior to the 1933 Long Beach earthquake practically all buildings built

in the State of California were designed for vertical loads only. In 1959

the Long Beach Building Code was amended to give the Building Department the

authority to condemn earthquake-hazardous buildings. Letters of condemnation

for 118 buildings and letters requesting demolition or repair for 149 buildings were sent by June 1969. After years of opposition by property owners,

on November 3, 1969, a moratorium period was declared by the City Council to

allow for further study of the provision for condemnation of earthquakehazardous buildings.

In July of 1971 these regulations were amended by a City Ordinance which

deals exclusively with the problem of existing hazardous buildings. Approximately 1,000 buildings built prior to the 1933 Long Beach earthquake came under this ordinance. As of May 1972 the Department has inspected six buildings and has sent all six owners of the buildings notices of excessive hazard.

The 1971 ordinance has the following features: 1) Priority for inspection of

pre-1933 buildings according to decreasing occupancy risk, and 2) Provides

owners' options to demolish, to strengthen the building for a given life, to

abandon and demolish the building within a specified time, to reduce the

occupancy risk to an acceptable level thereby possibly increasing the life

of the building, or combinations of the above. The 1971 ordinance is very

specific and covers such items as: 1) Dynamic Soil Condition Testing, 2)

Hazard Grading, 3) Calculation of Actual Lateral Force Withstanding Capability, 4) Minimum Tolerable Lateral Force Carrying Capability, 5) Owner's

Options, and 6) Procedures for Filing of Notices for Abatement. The Long

Beach program is a unique example of abatement of buildings declared to be

hazardous in the pre-earthquake period. The success of this program is due

almost entirely to continued efforts of a very dedicated building official,

Edward M. O'Connor, City of Long Beach. The 1971 ordinance was developed

based on the study by Wiggins and Moran [16], and the work of O'Connor [17].

The concept of a hazard grading system and the provision of options for the

building owners have considerable merit. The 1971 Santa Rosa ordinance is more straight-forward and is based on less theoretical methods of analysis,

and perhaps can be better understood by the building owners. It is recom- mended that these two 1971 ordinances be reviewed and a model ordinance

developed using the better portions of each ordinance.

Federal Agencies

Space limitations in this paper do not allow for a full discussion of

the pre-disaster plans for the survey and evaluation of existing Federal

buildings. A recent publication from the Office of Emergency Preparedness,

Disaster Preparedness , Vol. 1, Part IV [19] discusses this subject in detail.

Many Federal agencies, HUD, HEW, GSA, VA, NBS and the National Science

Foundation have varying responsibilities in this area. Federal agencies perform surveys of the buildings they own and lease to gather information on performance and maintenance needs, but the effort is neither comprehensive

nor continuous. Such activity is quite expensive, though no specific cost

data are available. These surveys are not "safety" surveys in the terms of

412
future disaster loss mitigation. Post-disaster surveys are performed on an ad hoc basis without proper criteria for evaluation of risk, and there have

been duplications of surveys by various Federal agencies in a given area. There is a vital need for these "safety" surveys and evaluations, as they represent one of the most effective means of extending regulation to

existing construction. One of the best methods of disaster loss mitigation

to existing structures is to require that there be an inspection and evaluation, prior to the granting of loans and other assistance for the alteration

of these structures.

Since the 1971 San Fernando earthquake, the Veterans Administration has

started an extensive program of investigation of the earthquake resistance

of their hospitals. They are starting with hospitals located in seismic

risk zones 2 and 3, Uniform Building Code. There are 81 V. A. hospitals in

these seismic risk zones. Replacement of these hospitals could cost several

billion dollars. The Administration has engaged a Board of Consultants which

includes experts in seismology, structural engineering and other aspects of

earthquake engineering. This Board of Consultants provides guidance on which

hospitals should be investigated, what type of special geological and soils

investigations should be made, and the procedures used to evaluate and recon- struct the older hospitals. Local architects and structural engineers are performing the actual investigation and evaluation of the hospitals as well

as preparing the plans and specifications for their reconstruction.

The Defense Civil Preparedness Agency (formerly "Office of Civil

Defense") in its National Fallout Shelter Survey has surveyed buildings to

establish fallout protection factors. The information for each investigated

building is coded on a "Fosdic" form which is read directly by the computer.

Unfortunately, the buildings in this survey usually do not have wood roof and

floor construction because of the low protection factors provided by these

light weight construction materials. These uninvestigated buildings would

include Type III buildings - Uniform Building Code - which have experienced

high damage losses during previous disasters, such as earthquake and wind.

Exterior and interior walls are coded according to their weight in pounds

per square foot, but the type of wall construction is not given. Also, the

type of construction is not given on the forms. Since the earthquake resistance of buildings is influenced primarily by the type of construction and

wall materials - McClure [7] - it would be difficult to rate the data com- piled on the Shelter Survey Forms.

Information is surprisingly lacking concerning the inventory of buildings

in the United States. The author's experience has been that except for an inventory of the one-to-four-unit dwellings given in the U.S. Census data, no inventory of buildings, even by occupancy, is readily available in the United

States. It is recommended that the Federal Government make a preliminary

study of the feasibility of gathering building inventory data by occupancy

and type of construction. It would be a tremendous task to gather the data

for all buildings, excluding dwellings, but this data is needed to provide

input data for disaster loss and loss mitigation studies.

A first step could include only emergency centers and other structures

with high socio-economic values that are located in identified high risk

land in the larger urban areas which are most vulnerable to natural disasters

[19].

VIII. Conclusions and Recommendations

Deficiencies exist in the state-of-the-art for the surveying and evaluation of the hazards represented by existing buildings and buildings damaged

by disasters. Current procedures used for the abatement of the hazards

413
represented by these buildings do not reduce these hazards to an acceptable

level of public safety. With the increased involvement of the Federal government in disaster loss indemnification programs, the Federal government has a vital interest in the development of new programs for disaster mitigation to

existing buildings. In the past this has been left almost entirely up to the

state and local governments, but this has resulted in an inconsistent approach

to the problem throughout the United States.

Improving building practices for disaster mitigation constitutes but one step in a more comprehensive approach required to abate hazardous existing

buildings and buildings damaged by disasters. Mitigation of the potential

disaster dollar losses to these buildings and the life hazard to the occupants

of these buildings must be based on: 1) Public recognition of the magnitude

of the hazard these buildings represent, 2) Characterization of the hazards

based on appropriate kinds of investigation, 3) Evaluation of the risk represented by these hazards, and 4) Alternative public policy decisions based on judgments of acceptable levels of risk and feasible abatement programs which

consider the social, political, and economic factors involved.

Federally sponsored programs must be based on:

1) Adequate inventory data on the number of existing buildings by

occupancy, type of construction, and other parameters which identify

their hazard potential.

2) Consistent criteria used to survey and evaluate the damage potential

represented by existing and damaged buildings.

3) Adequate disaster damage surveys that report reliable dollar loss

statistics.

4) Currently acceptable public policies expressed by laws and ordi- nances concerning abatement procedures.

5) The degrees of rehabilitation which can be achieved using present

repair procedures.

6) New knowledge which has developed from recent disasters, the

potential for new knowledge in future disasters, and the ability

to disseminate this new knowledge to the intended users including

the governmental agencies.

7) The need for research to evaluate current repair procedures and

research to develop new methods of repair.

8) Recognition of the political and economic considerations and other

problems in application of retroactive abatement procedures and

the lack of public policies to resolve these problems effectively.

9) Evaluation of alternative plans for Federal funding to provide

financial inducements for the abatement of hazardous buildings.

In the context of the above, the following conclusions and recommendations

for Federally sponsored studies are based on the findings presented in this

paper and the recommendations given in Steinbrugge, et al . (1970), "Earthquake Hazard Reduction" [14]. Although this latter report was directed to

the reduction of earthquake hazards, many of the recommendations are applicable to mitigation of losses from other natural disasters, such as hurricanes, tornadoes, and extreme winds.

• 1. There is a need for an inventory and data on all buildings that are located in potentially disaster hazardous urban areas of the United

414
states. The magnitude of the work of gathering these data is so large

that this information should first be gathered for buildings with high

socio-economic values - emergency centers, hospitals, and other occu- pancies with higher disaster support responsibilities. The appropriate

Federal agency should make a feasibility study of how to undertake such

an inventory which would provide data on the type of occupancy, type of

construction and other details of construction needed to make viable

potential risk evaluations. The magnitude of such an inventory could

exceed the National Fallout Shelter Survey program and, therefore, a feasibility study is recommended first rather than a recommendation to

undertake such an inventory study immediately.

2. There are no accepted criteria for the procedures to be used in

pre-disaster surveys and evaluations of existing buildings. The limited

number of such surveys have been conducted by various Federal, state,

and local agencies which has resulted in duplication and overlap.

Results of such surveys are not readily available nor even mutually

known. One Federal agency should be given the responsibility to develop

these criteria, publish guidelines, and study the feasibility of making

these reports more readily available. This work should be done in

cooperation with other Federal agencies which have statutory responsibilities to conduct such surveys and archive these reports.

3. Previous damage surveys have lacked adequate dollar damage statistics

reported in a consistent manner. It is too simplistic to recommend that

one "standard" form be developed for all disasters, but certainly the

present procedures can be improved using "Fosdic" type data forms to

gather, analyze, and report damage statistics. Processing of the data

should be done by computer at the local level as well as the Federal

level. Further study of the procedures used by Los Angeles County in

the San Fernando earthquake is recommended. Review of recently revised

inspection forms designed for computer analysis by the City of Los

Angeles is also recommended. The same Federal agency given the task

under recommendation 2 above should be given this responsibility.

4. There is no accepted criterion for the evaluation of the hazards

created by damaged buildings or by buildings constructed without adequate lateral force resistance. The Uniform Building Code's "Dangerous

Building Code," Part IV, 1967, represents one approach to this problem,

but the "Dangerous Building Code" is not generally adopted or enforced.

The appropriate Federal agency should develop criteria for such evaluations and develop model ordinances for adoption by local jurisdictions.

Approval of Federal funds for disaster loss indemnification and other

programs for the abatement of hazardous buildings should be continguent

upon adoption of such model ordinances. These ordinances should include

the new approaches now being used in Santa Rosa and Long Beach which

provide for various owner options. Important items in these ordinances

must be provisions to prevent the lowering of local safety standards

after the disasters and adequate monitoring of repair work by Federal

and local agencies. For such ordinances to be effective there must be

corresponding Federal and local government agencies to participate in

these abatement programs.

5. Improved building practices for the design and construction of

structural components are developed from theoretical non-testing studies,

the testing of these members and the observation of the performance of

these components in disasters. There is a need for more full-size scale

testing of structural members to better understand their failure mechanisms so that their degree of damage can be readily evaluated in damage

surveys. Most tests have been performed on small scale models and it is

only after disasters that opportunities are available to observe the

performance of full-size members when subject to yield point and failure

loadings. Unfortunately, immediate repairs to these members do not allow

415
for the proper evaluation of their performance during disasters. There

is a need to develop criteria based on full-sized tests for appropriate

guidelines to evaluate the levels of damage to structures and their

components which are not immediately obvious. One Federal agency should

be given the responsibility to develop test programs and to obtain

funding for testing of full-scale structural components from which the

above needed criteria can be developed.

t 6. Little testing or other research has been conducted concerning the

repair of damaged buildings. The methods of repair have not changed

appreciably except for the use of epoxy-type adhesives. It is necessary

to develop simple methods of repairing damaged structures based on tests

to insure continued acceptable levels of safety in the event of future

disasters. At present, the repair of damaged buildings is a relatively

new and untried art that has long been neglected by engineers. It is

left primarily to the skill and ingenuity of the engineer, on an ad hoc

basis, with little to guide him but his judgment. Repair of damaged

buildings has been performed on this basis without adequate test data

to verify the degree of rehabilitation attained. There is a vital need

for a program for the testing of structural members and their connections, which have been stressed above their yield point and then re- paired, to evaluate their resultant strength and ductility. One Federal

agency should be given the responsibility to review the adequacy of

present repair methods as well as to develop new methods of repair.

Testing programs should be developed and funded in cooperation with

construction material associations and structural engineers in private

practice. The test programs should include full-sized tests performed

on large shaking tables and in wind tunnels, and must include cyclic,

three dimensional dynamic loading as compared with the more traditional

simple reversal of loading procedures. This testing is too important to

be left to either the construction materials associations, the academic

theoretical researchers, the practicing structural engineers, the private

testing laboratories, or to the Federal government agencies, but must be

conducted using an inter-disciplinary approach. More effective procedures must be developed to make the new knowledge from such test programs

more readily available to the practicing engineers so that it can be more quickly incorporated in new codes and construction.

• 7. Immediate near term benefits in disaster mitigation will result from

the removal of unreinforced brick and masonry parapets, ornamentation,

improperly anchored exterior walls, and other appendages which represent

"clear and present" hazards in the event of earthquake and extreme winds.

A first step toward the mitigation of these hazards would be a survey

of buildings with these hazards in the high risk urban areas. Implementation of local programs, based on federally developed model ordinances, should be based on the procedures used in the City of Los

Angeles. Such programs must include a public education program combined

with a federally supported financial incentive program.

• 8. Viable programs for building hazard reduction must be based on cooperative Federal, state and local government disaster plans. The

survey and evaluation of damage immediately after the disaster is vital

to proper disaster mitigation. In the past there has been the traditional "patch and paint" approach which covers up the evidences of

damage prior to an adequate investigation of the damage. The appropriate

Federal agency should be given the task to see that the states have real

natural disaster response plans - not "paper" plans - and that these

plans are periodically tested as a condition for Federal funding of such

state programs. Review of the experience in the San Fernando earthquake

and the recent hurricanes is an obvious recommendation.

• 9. In the emergency period additional funds are needed for additional

inspectors, clerical and administrative staffing, for hiring of private

consulting engineers to make special studies, and for the use of special

416
equipment, such as helicopters and emergency communications systems.

It was not clear after the San Fernando earthquake whether or not funds

from Federal disaster programs were available for such needs and local

agencies were reluctant to spend their own money for these vitally

needed services. A re-evaluation of funds allocated to civil preparedness (civil defense) programs is needed. Apparently facilities and

equipment and equipment furnished through these programs are not available to local building departments during disaster emergency operations.

Pre-disaster funding for emergency, portable hand-operated two-way radio

communications for building departments should be given the highest

pri ori ty

.

t 10. After previous earthquakes and other disasters there has been a lack of coordination between Federal agencies which have statutory

responsibilities or interests in making immediate surveys of the damage

during the emergency period. This has resulted in duplication and

incomplete reporting by some agencies. Their studies have been "mission"

oriented, performed too late, and the new knowledge which develops out

of each disaster is lost or not made readily available to the state and

local agencies or to those in the construction industry who could benefit

from these early studies. One appropriate Federal agency should be given

the responsibility to develop pre-disaster response plans for the various

Federal agencies and non-governmental institutions to make meaningful

immediate investigations of the damage. Federal funds should be made

available on a stand-by basis to provide for adequate studies and preparation of reports for distribution at the earliest practical date.

These investigations should be coordinated to cover all aspects of the

disaster and require a mu 1 ti -di sci pi i n ary oriented approach. Hopefully,

only a few comprehensive reports would be prepared to minimize the

duplication and number of different "mission" oriented reports. The use

of preliminary reports is strongly recommended to expedite the dissemination of the new knowledge to those who have an immediate need for this

i nf ormati on

.

• 11. Development and implementation of disaster mitigation programs by

the Federal Government can take place only if there are accepted public

policies at all levels of government supporting such programs. The

public is generally not fully informed concerning the possibility for

and the consequences of natural disaster until after they have occurred.

A coordinated Federal Government program is needed for public education

on a continuing basis concerning the potential damage which can be

caused by natural disasters and what can be done to minimize their

effects. An informed public will aid in making and supporting public

policy decisions which weigh the economic and social impact and life

hazard posed by these disasters against the cost of reducing these

hazards to an acceptable level of risk. It is recommended that an increased public education program concerning disasters and disaster

mitigation possibilities be undertaken by a Federal agency with adequate

funds for continuing implementation by all levels of government.

IX. References

1. California Joint Committee on Seismic Safety Advisory Group on Disaster

Preparedness, Minutes, August 18, 1971.

2. Structural Engineers Association of California, Proceedings of the 40th

Annual Convention, October 7, 8, and 9, 1971. "Reports by Building

Agencies - Immediate Actions and Permanent Repairs." E. T. Brown, et al

.

Pages 68-77.

417
3. Steinbrugge, K. V., et al . , (1970) "The Santa Rosa, California, Earthquakes, October 1, 1969." Environments Sciences Services Administration,

U.S. Department of Commerce.

4. Steinbrugge, K. V., et al . (1964) "The Alaska Earthquake, March 27, 1964."

National Board of Fire Underwriters and Pacific Fire Rating Bureau.

5. Office of Emergency Planning (1964) "The Alaska Earthquake, A Progress

Report - 279 Days of Federal Reconstruction Effort."

6. Steinbrugge, K. V. and D. F. Moran (1954). "An Engineering Study of the

Southern California Earthquake of July 21, 1952 and Its Aftershocks."

Bull. Sei smol ogi cal Society of America, Vol. 44, 2B , pp. 199-462.

7. McClure, Frank E. (1967). "Studies in Gathering Earthquake Damage

Statistics." ESSA - Coast and Geodetic Survey.

8. Earthquake Engineering Research Institute (1964). "Earthquake Damage

Survey Guide." Damage Analysis Committee.

9. Structural Engineers Association of Northern California. "Subcommittee

on Repair of Earthquake Damage." Report submitted by Leslie W. Graham,

et al . , August 20 , 1 970 , to Board of Directors.

10. Hanson, R. D. and H. J. Degenkolb (1969). "The Venezuela Earthquake,

July 29, 1967." American Iron & Steel Institute, New York.

11. Uniform Building Code, 1970 edition.

12. Uniform Building Code, "Dangerous Buildings." Volume IV, 1967 edition.

13. Degenkolb, H. J. (1971). "Earthquake Safety Investigation Criteria for

City of Santa Rosa, California." (Private Paper.)

14. Steinbrugge, K. V. (Chairman) et al . , ( 1 970). "Task Force on Earthquake

Hazard Reduction." Office of Science and Technology, Executive Office

of the Pres i dent

.

15. Steinbrugge, K. V. (1968). "Earthquake Hazard in the San Francisco Bay

Area: A Continuing Problem in Public Policy." Institute of Governmental

Studies, University of California, Berkeley.

16. Wiggins, J. H., Jr. and D. F. Moran (1971). "Earthquake Safety in the

City of Long Beach Based on the Concept of Balanced Risk."

17. Structural Engineers Association of California, Proceedings of the 39th

Annual Convention, October 15-17, 1970. "Pre-Field Act Buildings in the

City of Long Beach." E. M. O'Connor. Pages 20-24.

18. Joint Committee on Seismic Safety on the California Legislature (1971).

"Earthquake Risk." October 31, 1971 Interim Report of the Special

Subcommittee on the San Fernando Earthquake Study.

19. Office of Emergency Preparedness (1972). "Disaster Preparedness."

Vol. I, II, & III. Report to the Congress.

20. National Bureau of Standards (1971). "Engineering Aspects of the 1971

San Fernando Earthquake." Building Science Series 40.

21. U.S. Army Corps of Engineers, Waterways Experiment Station (1969).

"Hurricane Camile, Damage Survey 1969." Draft copy.

418
22. Mehta, K. C. and A. J. Sanger (1971). "Response of Structural Systems

to the Lubbock Storm." Texas Tech University, Report TTU SRR,03.

23. Reynolds, G, W. (1958). "Tornadoes and How to Handle Them." Consulting

Engineer, May 1958.

24. Factory Mutual Engineering Division (1959). "Wind Forces and Roof

Anchorage Design." Chapter 69, McGraw-Hill Book Company, New York.

25. National Forest Products Association (1972). "How to Build Storm

Resistant Structures." 1619 Massachusetts Avenue, N.W., Washington,

D.C. 20036.

26. U.S. Forest Service (1969). "Wood Structures Survive Hurricane Camille's

Winds." Forest Service Research Paper FPL 123, October 1969.

27. Anderson, L. 0., and W. R. Smith (1965). "Houses Can Resist Hurricanes."

U.S. Forest Service Research Paper FPL 33, August 1965.

28. State of California, Education Code Part 3 - Division 11 - Chapter 2

- Article 4, "Field Act." Section 15451 through 15465.

29. State of California, Education Code, Part 3 - Division 11 - Chapter 2 - Article 5, "Garrison Act." Section 15503 through 15518.

Selected Bibliography

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Binder, R. W. (1952). "Engineering Aspects of the 1933 Long Beach Earthquake."

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Carder, D. S., Editor (1965). "Earthquake Investigations in the Western

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Degenkolb, H. J. Committee Member (1951). "Lateral Forces of Earthquake and

Wind." Trans. ASCE, Vol. 117, pp. 715-780.

Hanson, R. D., and H. J. Degenkolb (1969). "The Venezuela Earthquake,

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Jennings, P. C, Editor (1971). "Engineering Features of the San Fernando

Earthquake, February 9, 1971." California Institute of Technology,

Pasadena

.

Joint Technical Committee on Earthquake Protection (1933). "Earthquake

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Los Angeles County Earthquake Commission (1971). "San Fernando Earthquake,

Feb ruary 9 , 1971.

Moran, D. F., Editor (in press). "Report on the San Fernando Earthquake,"

Earthquake Engineering Research Institute - National Oceanic and

Atmospheric Administration.

McClure, F. E. (1967). "Studies in Gathering Damage Statistics." ESSA - Coast and Geodetic Survey.

McClure, F. E. (in press). "Performance of Single Family Dwellings in the

February 9, 1971 San Fernando Earthquake." HUD - National Oceanic and

Atmospheric Administration.

419
National Academy of Sciences (1970). "The Great Alaska Earthquake of 1964:

Human Ecol ogy

.

National Board of Fire Underwriters (1933). "Report on the Southern

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)

Office of Emergency Preparedness (1969). "Geologic Hazards and Public

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State of California Resources Agency (1964). "Earthquake and Geologic Hazards

Conference." December 7-8, 1964, San Francisco.

State of California Resources Agency (1967). "Earthquake and Geologic Hazards

in California." Geologic Hazards Advisory Committees for Program and

Organization, April 26, 1967.

Steinbrugge, K. V. (Chairman) et al . , (1970). "Task Force on Earthquake

Hazard Reduction." Office of Science and Technology, Executive Office

of the President.

Steinbrugge, K. V. and D. F. Moran (1954). "An Engineering Study of the

Study of Eureka, California, Earthquake of December 21, 1954." Bulletin

of the Seismological Society of America, Vol. 47, pp. 129-153.

Steinbrugge, K. V., E. E. Schader, H. C. Bigglestone, and C. A. Weers (1971).

"San Fernando Earthquake, February 9, 1971." Pacific Fire Rating Bureau,

San Francisco.

Steinbrugge, K. V., and E. E. Schader (in press). "Earthquake Damage and

Related Statistics." National Oceanic and Atmospheric Administration

(volumes on the San Fernando earthquake).

Steinbrugge, K. V., E. E. Schader, and D. F. Moran (in press). "Building

Damage in San Fernando Valley." California Division of Mines and

Geology, Sacramento, California.

Steinbrugge, K. V. (1968). "Earthquake Hazard in the San Francisco Bay

Area." Institute of Governmental Studies.

Steinbrugge, K. V. (1970). "Engineering Aspects of the Santa Rosa, California,

Earthquakes, October 1, 1969." ESSA, U.S. Department of Commerce.

Steinbrugge, K. V. et al . , ( 1 970). "Earthquake Hazard -Reducti on . " Executive

Office of the President, Office of Science and Technology.

Steinbrugge, K. V., F. E. McClure, and A. J. Snow (1969). "Studies in

Seismicity and Earthquake Damage Statistics, Appendix A." ESSA - Coast

and Geodetic Survey.

Steinbrugge, K. V., and V. R. Bush (1965). "Review of Earthquake Damage in

Western United States, 1933-1964." Earthquake Investigations in the

Western United States, 1931-1964, pp. 22-256.

Structural Engineers Association of California (1968). "Recommended Lateral

Force Requirements and Commentary."

420
U.S. Coast and Geodetic Survey (1965). "The Puget Sound, Washington Earthquake of April 29, 1965." U.S. Government Printing Office, Washington,

D.C.

U.S. Coast and Geodetic Survey. "The Prince William Sound, Alaska, Earthquake of 1964 and Aftershocks." ESSA, U.S. Government Printing Office,

Washington, D.C.

U.S. Geological Survey and National Oceanic and Atmospheric Administration

(1971). "The San Fernando, California Earthquake of February 9, 1971."

A preliminary report. U.S.G.S. Professional Paper 733.

Publications - Other Natural Disasters

Anderson, L. 0. and W. R. Smith (1965). "Houses Can Resist Hurricanes."

U.S. Forest Service Research Paper FPL 33, August 1965.

ASCE Task Committee on Wind Forces (1961). "Wind Forces on Structures."

American Society of Civil Engineers Transactions, Vol. 126, Part II,

1961 , pp. 1 1 24-11 98.

Biggs, John M. (1958). "Wind Forces on Structures: Introduction and

History." Journal of the Structural Division, ASCE, Vol. 84, No. ST4,

July 1958, Paper 1707.

Carney, J. M. (1966). "Hurricane Resistant Plywood Construction." ASCE

Structural Engineering Conference, Miami Beach, Florida, January 31-

February 4 , 1 966 .

Civil Engineering (1961). "Lessons Learned from Hurricane Donna, Florida,

September 10 and 11, 1960." March 1961.

Civil Engineering (1960). "Miami Suffers Slight Structural Damage in Severe

Hurricane, October 17, 1950." December 1960.

Engineering News Record (1970). "Major Buildings Survive Tornado That Does

$200-minion Damage." McGraw-Hill (New Jersey), May 21 , 1 970 , p. 35.

Engineering News Record (1970). "Tornado Skews 21-story Steel Frame Building."

McGraw-Hill (New Jersey), July 9, 1970, p. 19.

Factory Mutual Engineering Division (1959). "Wind Forces and Roof Anchorage

Design." Chapter 69, McGraw-Hill Book Company, New York.

Howe, George E. (1934). "Building to Resist Hurricanes in the West Indies."

American Institute of Steel Construction.

Kennedy, W. D. (1967). "Some Preliminary Observations cn a Hospital Response

to the Jackson, Mississippi, Tornado of March 3, 1966." The Disaster

Relief Research Center, Ohio State University, Research Report 17.

Marshall, R. D. and H. C. S. Thom, Editors (1970). "Wind Loads on Buildings

and Structures." Building Science Series 30. U.S. Department of

Commerce, National Bureau of Standards.

McDonald, James R. (1970). "Structural Response to a Twenty-Story Building

to the Lubbock Tornado." Storm Research Report 01, Texas Tech University

(Lubbock) , October 1970

.

Mehta, K. C. and A. J. Sanger (1971). "Response of Structural Systems to the

Lubbock Storm." Texas Tech University, Report TTU SRR 03.

421
Minnis, Mhyra and S. Perry McWilliams (1971). "The Voice of People in

Disaster and After: A Study in Residential Integration." Texas Tech

University (Lubbock), September 1971.

Minor, Robert R. and Albert J. Sanger (1971). "Observations of the Response

of Metal Building Systems to the Lubbock Tornado." Texas Tech University

Storm Research Report 02, Texas Tech University (Lubbock), February 1971.

National Forest Products Association (1972). "How to Building Storm Resistant

Structures." 1619 Massachusetts Avenue, N.W., Washington, D.C. 20036

Office of Civil Defense (1970). "Lubbock Tornado." Office of Civil Defense

Report, Region 5, May 1970.

Office of Emergency Preparedness (1972). "Disaster Preparedness," Vol. 1,

Part III: Disaster Protection, Chapter C, "Tornadoes and Windstorms,"

Chapter D, "Hurricanes and Storm Surges," and Part IV: Disaster Mitigation. Report to the Congress.

Otsaki, Yukio, M. S., (1967). "Tall Buildings and Areoelastic Problems:

Earthquake and Wind." Executive Director, Shimizu Construction Co.,

Ltd.

Parmelee, Richard A., Editor (1970). "Wind Effects on High-Rise Buildings."

Proceedings of the Design Symposium held at Northwestern University,

Evanston, Illinois, 1970.

Reardon, L. F. (1926). "The Florida Hurricane and Disaster." Miami Publishing Company, Miami, Florida.

Reynolds, G. W. (1958). "Tornadoes and How to Handle Them." Consulting

Engineer, May 1958.

Somes, Norman, Robert D. Dikkers, and Thomas H. Boone (1970). "Lubbock

Tornado: A Survey of Building Damage in an Urban Area." National

Bureau of Standards Report 10254, Project 42100-4215110, June 22, 1970.

U.S. Army Corps of Engineers, Waterways Experiment Station (1969). "Hurricane Camille, Damage Survey 1969" - draft copy.

U.S. Department of Commerce, NOAA (1971). "Mississippi Delta Tornadoes of

February 21, 1971." Natural Disaster Survey Report 71-2.

U.S. Forest Service (1969). "Wood Structures Survive Hurricane Camille's

Winds." Forest Service Research Paper FPL 123, October 1969.

U.S. - Japan Panel on Wind and Seismic Effects (1970). Second Joint Meeting,

Washington, D.C, May 1 3-15 , 1 970.

University of Toronto (1968). "Wind Effects on Buildings and Structures."

Vols. I and II, International Research Seminar, September 11-15,

University of Toronto Press.

Whitehead, C. J., B. K. Lambert and J. E. Minor, Editors (1971). "A City's

Response to Disaster." Texas Tech University Storm Research Report 04,

Texas Tech University (Lubbock), October 1971.

Wittman, John and J. E. Minor (1970). "Response to Manufacturing Systems to

the Lubbock Storm of May 11, 1970." Texas Tech University Storm Research

Report

.

422
Publications - Repair

Champion, S. (1961). "Failure and Repair of Concrete Structures." John

Wi 1 ey & Sons .

Foth , U. ( 1 972 ). "Earthquake Damage Repair Techniques." Report on the San

Fernando Earthquake, Earthquake Engineering Research Institute - National

Oceanic and Atmospheric Administration. (In Press)

Hunter, L. E. (1952). "Underpinning and Strengthening of Structures." Contractors Record and Municipal Engineering, Lennox House, Norfold Street,

W.C. London, England.

Johnson, Sidney M. (1965) "Deterioration, Maintenance, and Repair of Structures." McGraw-Hill Book Company.

McKaig, Thomas H. (1962). "Building Failures." McGraw-Hill Book Company.

Codes, Laws, Ordinances and Regulations

City of Bakersfield, October, 1952. Emergency Ordinance No. 956, New Series,

An Emergency Ordinance Amending Table 24-E, Ordinance Sections 2524 and

2702 (a) of Ordinance No. 909, New Series.

City of Los Angeles Building Code (1970).

County of Los Angeles Building Code (1968).

Joint Committee on Seismic Safety and Advisory Groups on Engineering

Considerations and Earthquake Sciences, Governmental Organization and

Performance, Disaster Preparedness, Land Use Planning and Postearthquake Recovery and Redevelopment. June 30, 1970 Progress Report.

Ibid. July 31, 1971 Preliminary Report on the San Fernando Earthquake Study.

Long Beach Municipal Code - Ordinance No. C-4950, July 30, 1971. An

Ordinance Amending Ordinance No. C-3600 By Adding Thereto Subdivision

80, Consisting of Sections 8100.8000 Through 8100.8014, Pertaining to Earthquake Hazard Regulations for Rehabilitation of Existing Structures

Within the City.

Public Law 90 - 606, 91st Congress, S. 3619. December 31, 1970.

San Francisco Building Code, March 20, 1969. Section 203.0 to 203. l.F

"Serious and Immediate Hazards." pp. 30-33.

San Francisco Building Code, March 20 , 1969. Section 104. B.l. "Lateral

Force Provision, Existing Buildings." Riley Act.

Santa Rosa Resolution No. 9820, October 12, 1971. A Resolution of the City,

establishing Criteria for the Inspection of Buildings within the City

of Santa Rosa.

State of California, Civil Defense and Disaster Plan Plus Amendments 1967,

1968, and 1969.

Uniform Building Code, 1967, 1970 editions. Sections 104 and 203.

Uniform Building Code, "Dangerous Buildings." Volume IV, 1967 edition.

423
Disaster Plans and Damage Surveys

County of Los Angeles, Department of County Engineers, Building and Safety

Division, June 20, 1969, Damage Inspection and Reporting Procedures.

Draft A, Engineering Annex, Emergency Operations Plan, City and County of

San Francisco.

Draft A, Standard Operating Procedure, Engineering Division, Emergency

Operations Plan, Disaster Corps, City and County of San Francisco,

September 26 , 1969 . Earthquake Engineering Research Institute (1964). "Earthquake Damage Survey

Guide." Damage Analysis Committee.

"Engineer Service." Emergency Operations Plan, State of California Disaster

Office, April 7, 1961

.

Federal Reconstruction and Development Planning Commission for Alaska (1964)

"Response to Disaster, Alaskan Earthquake, March 27, 1964."

Los Angeles Department of Building and Safety, Disaster Orders, December 27,

1967.

Los Angeles Department of Building and Safety, Disaster Orders, Draft,

February 7, 1972.

Los Angeles County and Cities, Disaster Relief Manual, Addendum, Earthquakes

Office of Emergency Preparedness (1972). "Disaster Preparedness." Vol. I,

II, & III. Report to the Congress.

Office of Emergency Planning (1967). "Report of Earthquake Seminar."

September 1967, Region Seven, Santa Rosa, California.

Office of Emergency Preparedness (1971). "Q-Day + 100 - Federal Response to

the California Earthquake of February 9, 1971." U.S. Government Printi

Office, Washington, D.C.

"Plan Bulldozer." Disaster Relief, Northern and Central California Associated General Contractors.

Special Subcommittee of the Joint Committee on Seismic Safety, California

Legislature, July 31, 1971. "Preliminary Report on the San Francisco

Earthquake Study." Sacramento, California, August 1971.

U.S. Senate, Governmental Response to the California Earthquake Disaster of

February 9, 1971. Hearings before the Committee on Public Works, 92nd

Congress. First Session, 1971.

American Papers (Unpublished)

Bush, Vincent R., "Building Department Operations." Unpublished paper,

September 1971 . Degenkolb, H. J. (1971). "Earthquake Safety Investigation Criteria for City

of Santa Rosa, California."

Degenkolb, H. J., and Associates, Graham & Hayes, McClure & Messinger, et al "Relative Earthquake Hazards of the Pre-Field Act School Buildings with

the San Francisco Unified School District." Report to the Director

Works, San Francisco, August 10, 1971.

424
Meehan, J. F. Notes of trip to Reno, Nevada, "To Observe Effects of Strong

Wind on Structures." Office of Architecture and Construction, Department of General Services, State of California.

Oakland Unified School District (1970). "Estimated Cost for Replacement or Reconstruction of Non - Con f ormi n g School Buildings." James F. Fisher,

Kent Friel , and Frank E. McClure.

O'Connor, E. M. (1972). "Existing Earthquake Hazards."

Olson, Robert (1970). "From the Viewpoint of Earthquake Safety: Directions

for Public Policy." Institute of Governmental Studies, University of

California, Berkeley, California.

Recommendations, Joint Committee of Redwood Empire Branch of American Society

of Civil Engineers and Structural Engineers Association of Northern

California for Santa Rosa Earthquake Evaluation, October 27, 1969.

Wailes, C. D. , Jr. and A. C. Horner (1933). "Survey of Earthquake Damage at

Long Beach, California."

Foreign Papers

Leaflets issued by the Building Research Station, Department of Scientific

and Industrial Research, Garson, England, 1944. (Issued in anticipation

of the end of the war.)

Leaflets issued by the Greek Government on the Repair and Reconstruction of

Buildings after the 1965 Earthquake by the Ministry of Public Works,

Athens, General Directorate of Public Works, Housing Service.

Ohka, D. (1929). "The Reconstruction Work in Tokyo and Yokohama." Proceedings of World Engineering Congress, Tokyo, IX, pp. 1-24.

Report on a Seminar on Strengthening Earthquake-Damaged Buildings in San

Salvador, Central America. Liverpool, July 10-24, 1965, sponsored by

the Ministry of Overseas Development, United Kingdom.

Society of Earthquake & Civil Engineering Dynamics, "Effects of Earthquake

and Disaster on the Resident Population and the Work of Rescue Teams."

Octover 1 , 1 969

.

Society of Earthquake & Engineering Dynamics. "Repair of Earthquake of

Damaged Buildings." September 1969.

Susinno, Alessandre (1929). "Rules of Construction and Legislation in

Regions Striken by Earthquake (Italy)." Proceedings of World Engineering Congress, Tokyo, VIII, pp. 313-325.

UNESCO (1961). Selections from "Asia Seismology and Earthquake Engineering

Semi nar

. " Tokyo

.

Other Bibliography

Degenkolb, H. J. (1951). "Lateral Forces of Earthquake and Wind." Trans.

ASCE, Vol . 117, pp. 716-780.

General Services Administration (1968). "Buildings Bibliography." Compiled

by Central Office Library, Office of Administration.

Mollis, E. P. (1971). "Bibliography of Earthquake Engineering." Third

edition. Earthquake Engineering Research Institute, Oakland, California.

425
Office of Emergency Preparedness (1972). "Disaster Preparedness." Report

to the Congress, Volumes 1 and 3.

X. Acknowledgments

The scope of this paper is so broad that it could not be written based

on the knowledge of one person. The author is grateful to Leslie W. Graham,

Chairman of the SEAONC Committe on "Emergency Earthquake Damage Survey

Procedures and Guidelines for Earthquake Damage Repairs," who provided his

complete committee file for use in preparation of this paper. Henry J. Degenkolb made available the use of his extensive library. Karl V. Stei nbrugge ' s writings provide a substantial resource. Hopefully, the cited

references and selected bibliography give proper credit to the many others

who provided valuable input.

The author is appreciative of the review comments by the SEAOC Seismology Committee members, John F. Meehan, Thomas D. Wosser, and Hans 6.

Steinmann. The final formating and typing of the paper was done under the

direction of William W. Giles, Project Administrator, Applied Technology

Council. Finally, the author wishes to express his appreciation to the

Applied Technology Council, the National Science Foundation and the National

Bureau of Standards who provided the financial support to prepare and present

this paper.

National Bureau of Standards Building Science Series 46,

Building Practices for Disaster Mitigation , Proceedings

of a Workshop Sponsored by the National Science Foundation

and the National Bureau of Standards, August 2 8-September 1,

1972, Boulder, Colorado (Issued February 1973).

426
ABNORMAL LOADING ON BUILDINGS AND PROGRESSIVE COLLAPSE

by

Norman F. Somes

I . Introducti on Since 1968, there has been growing international concern that multistory

buildings are frequently designed without explicit consideration for abnormal

loading conditions. On May 16 of that year, there occurred the much-publicized collapse of apartments in the Ronan Point building in London. The

building has 22 stories of precast concrete panel construction above a castin-place concrete podium. A typical floor layout is shown in Figure 1 in

which the structural walls are shown solid. The collapse was triggered by

an accidental explosion of gas that leaked from the connection of a gas range

located in an apartment on the 18th floor. The Report of the Inquiry into the

Collapse [1] states:

"The explosion blew out the non- 1 oad-beari ng face walls of the

kitchen and living room, and also, unfortunately, the external

load-bearing flank [end] wall of the living room and bedroom of

the flat, thus removing the support for the floor slabs on that corner of the nineteenth floor, which collapsed. The flank walls

and floors above this collapsed in turn, and the weight and impact

of the wall and floor slabs, falling on the floors below, caused a progressive collapse of the floor and wall panels in this corner

of the block [building] right down to the level of the podium."

The podium and the extent of the collapse is shown in Figure 2; an adjacent

building of identical construction shows the appearance of the building

prior to the collapse. From Figures 1 and 3 it may be seen that collapse

affected both the living room and bedroom above the 16th floor, while below

this level, the collapse was limited to the living room. Four people were killed in the collapse and seventeen people were injured.

"The loss of life and injury might well have been very much worse.

At 5:45 a.m., mercifully, most tenants were in their bedrooms . . ."

The Report also documents that, by a fortunate chance, of the four apartments

directly above the one in which the explosion took place, only one was occupied

at that time.

The Report of the Inquiry drew international attention to several deficiencies in existing codes and standards, particularly as they applied to

multistory buildings. Interim additional criteria [2] were quickly implemented

in the United Kingdom having regard to the appraisal and strengthening of

existing buildings and the design of new structures. Several other countries

in Europe introduced additional design criteria to explicitly deal with the

risks exposed by the Ronan Point incident.

To date, with one exception, the U.S. codes- and s tandards -wri t i n g bodies

have not published criteria taking account of abnormal loads and progressive

collapse. The exception is the 1972 American National Standards Institute

A58, Minimum Design Loads in Buildings and Other Structures [3] which provides

a short statement drawing the attention of the designer to the problem.

Several standards-writing bodies have established technical committees to

consider the problem, however.

427

493-610 O - 73 - 29
Figure 1 Typical floor layout of Ronan Point apartment

building (explosion occurred in a SE-corner

apartment). Reproduced from [1].

428
Figure 2 Ronan Point apartment building after the

collapse, with a second identical building

in the background. Reproduced from [1].

429
Figure 3 Layout of the SE-corner apartment on the 18th

floor where the explosion originated,

- Reproduced from [1].

430
The prime mover in the matter of progressive collapse in the USA has,

to date, been the Department of Housing and Urban Development (HUD). In August 1969, provisions against progressive collapse were included for the

first time in a Structural Bulletin [4] issued to the producer of a precast

concrete housing system by HUD's Federal Housing Administration (FHA).

Criteria against progressive collapse were included in HUD's Guide Criteria

documents [5] prepared by the National Bureau of Standards (NBS) early in

1970 and implemented in HUD's experimental housing program Operation Breakthrough. In October of 1971 the FHA circulated its draft document "Provisions

to Prevent Progressive Collapse" [6] for review by certain trade associations,

standards-writing bodies and members of the design profession. This document

expressed criteria intended for application by FHA in evaluating multistory

buildings for which Federal mortgage assurance is required. The document has

not been circulated in final form but the draft has served as a starting point

for much discussion. In each of these three preceding instances, the criteria

reflected the United Kingdom requirements at that time.

In November 1971, the National Bureau of Standards, at the request of

HUD, commenced a detailed study of abnormal loading on buildings in the USA

and the many aspects pertinent to the avoidance of progressive collapse. The

results, to date, provide the basis for this interim progress report, which

firstly attempts to relate the many parameters of the problem, to classify

and discuss the various sources of abnormal loading and to quantify their

frequency insofar as the currently available U.S. data permits. The implication of these findings for the USA are then discussed. The response of

buildings and building elements to abnormal loading is then reviewed, including cases of progressive collapse. Several alternative approaches for the

introduction of criteria are presented and, finally, conclusions are given

with respect to the problem posed in the U.S.A.

II . Problem Defini tion

Progressive collapse may be defined as a chain reaction of failures

following damage to a relatively small part of a structure .

An abnormal loading may be defined as a condition of loading which a designer, following established practice, does not include in the normal

analysis and design of a particular structure . It is a loading condition of

sufficient severity and probability of occurrence to be a cause for concern,

but still of such a relatively rare nature as to be outside of normal designlife expectancy. This definition goes beyond that of static and dynamic

forces and includes such conditions as the dislodgement of a bearing wall

panel, and the development of a weld failure in a steel connection.

Recent reports [7, 8] confirm the growing view that studies of the

problem of progressive collapse should deal generally with multistory con- struction and not be limited to high-rise construction or simply that using

precast concrete panels. However, the view is widely held that framed

buildings are more tolerant of local damage and have more resistance to

progressive collapse than load-bearing structures. It is reasoned that this

is due to the fact that continuity between members is more easily accomplished

in framed buildings than in load-bearing structures, and that the former have

greater ability for developing alternate paths for forces in the event of the

loss of a critical member. This viewpoint is supported by the documented

experience of engineers during World War II bombings [9].

Occupants of multistory buildings, whatever type of construction is

used, have a right to expect adequate and consistent levels of safety. The user requirement may be expressed as an adequate protection from extreme

loads . Expressed as a performance requirement, this corresponds to adequate

strength namely, compliance, with a specified load capacity. Present U.S.

431
design standards specify load capacity in terms of severe combinations of

dead, live, snow, wind, or earthquake loads. They do not specify load

capacity with respect to abnormal loading conditions.

In a general sense, adequate safety is achieved by insuring that, at

loads less than the specified load capacity, there is no loss of static

equilibrium resulting in:

Local Col 1 apse , or - Extens i ve Collapse.

The Ronan Point incident was clearly due to an abnormal loading condition and the collapse was extensive. Had the damage been confined to the

apartment in which the explosion originated, it is doubtful whether the

accident would have received more than local newspaper coverage. Such

explosions occur somewhere everyday and arouse little reaction from society

at large. It is only when they produce extensive collapse that they generate

international attention.

The foregoing discussion serves to delineate the NBS study which is

concerned with abnormal loadings on multistory buildings of any type of

construction, and the need to prevent progressive collapse .

III. Public Acceptance of Risk

Risk is a function of the probability of occurrence and the consequence

of a particular event. Zero risk in the face of all possible conditions and

hazards can never be achieved. By assessing the statistics of all foreseeable

hazards and evaluating their consequence, an acceptable level of safety can

be achieved, acceptable safety at an acceptable cost. The acceptable risk to

life and property is probably best decided by representatives of the community

at large.

Otway et al [10] have provided one basis for considering the risks with

which society is prepared to live. For this purpose, they use the U.S.

accidental death statistics for 1966. The probability of death per person per

year is given for a series of types of accidents in the following table:

Table 3.1

Motor Vehicle 2 .7 x 10-4

Falling 1 ,.0 x 10-4

Fi re 4,.0 X 10-5

Drowni ng 2 .8 X 10-5

Fi rearms 1 .3 X 10-5

Poi soni ng 1 ,.1 X 10-5

Earthquake 8,.0 X 10-7

Lightni ng 5 .5 X 10-7

432
Tlie paper points out that situations providing hazards on the order of

10"^ deaths per person per year are uncommon. When a risk approaches this

level, immediate action is taken to reduce the hazard. This level of risk

is unacceptable to everyone in society. At an accident level of 10"^ deaths

per person per year, people spend money, especially public money, to control

the cause. Risks at the level of 10"^ deaths per ygar are still significant

to society. Accidents with a probability about 10"° deaths per person per

year are not of great concern to the average person. He may be aware of them

but he feels they will never happen to him. There is a general lack of con- cern about accidents having a mortality risk of less than 10'^ per person per

year.

Some qualification should be made with respect to this last point and

the statistic for earthquakes. In fact, a considerable amount of money is

spent on earthquake protection and this would appear to conflict with Otway's

last general comment. However, Otway divided incidents by the total U.S.

population whereas the earthquake risk is geographically concentrated. If,

for example, the population living within seismic Zone 3 were used, the

probability of death per person would be increased by a factor of approximately 5, to 4.0 X 10-6, thereby appearing to remove the above conflict.

Another factor contributes to the considerable U.S. expenditure on earthquake

protection; in addition to deaths and injuries, serious earthquakes are accompanied by large property losses.

Public expenditure to provide protection against fire in buildings is

very considerable and this is in response to a probability of 4 x lO'^ deaths

per person per year together with heavy property damage. It would be useful

to assess the frequency of abnormal loading incidents that, for people living

in buildings susceptible to progressive collapse, would constitute a risk of

death from that cause equal to that due to fire. It is important to differentiate between buildings that are susceptible and those not susceptible to

progressive collapse. Figure 4 shows a speculative plot of the total number

of U.S. housing units, increasing with time, and having a value of Y units at

a particular point in time. There is a reason to believe that, in the absence

of new criteria to minimize the risk of progressive collapse, the number of

buildings that are susceptible, will otherwise increase. A second plot shows

the number of susceptible units increasing with time from a relatively insignificant level to a very significant fraction of the total number of housing

units. Of course, it is with the objective of stopping any such growth, that

the NBS study and related studies are underway. Nonetheless, it is assumed

that, in the absence of specific criteria, the number of housing units sus- ceptible to progressive collapse have risen to X at the point in time previously

referred to, X units out of total of Y.

The following assumptions will be made in order to carry out a comparison

of the respective risks due to progressive collapse and fire:

1. The total number of abnormal loading incidents affecting U.S.

housing units in a year corresponding to the selected point in

time is N.

2. The probability of occurrence of an abnormal loading on a housing

unit is the same irrespective of the type or location of the unit.

3. The abnormal loadings in question are of a severity that in the case of susceptible buildings, progressive collapse could occur.

On the basis of assumption 2, the probability of an abnormal loading per year

in a housing unit susceptible to progressive collapse is N/Y.

In what follows, the result will be seen to depend upon the size and

architectural arrangement of the multistory building considered. Consider a 100 housing unit building with a central service core, four housing units per

433
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434
story and an average occupancy rate of 4 persons per unit. If as in the

Ronan Point building, the collapse affected one quarter of the building in plan, then an estimate of life loss would have to be based upon some fraction of the number of occupants of that quarter. For part of each

24 hours, people will be absent from the building e.g., at work or school or in recreational activity. Furthermore, it is doubtful if more than one-half of the casualties would result in death. There would be a number of severely injured, less injured, etc. When both factors are considered, a reasonable estimate of the number of deaths would appear to be 25, namely 1/16 of the

total number of building occupants.

The following discussion will be confined to units susceptible to progressive collapse.

1 s The probability of an abnormal loading in a 100 unit building per year

y- X 100

The risk of death due to progressive collapse per person per year in

that 100 unit building is

^ X 100 X

Y 1

6

Equating this risk of fatality to that due to fire

N 1 ,-5

y- X 100 X yg- 4 X 10

If Y is taken to correspond to the U.S. housing unit total as given by the

1970 U.S. Housing Census [11], namely 67.7 million

N

= 4 X 10"^ X 16 X 67.7 x 10^

100

= 433 incidents

This result states that, for the conditions considered, an annual U.S.

total of 433 abnormal loadings on housing units would result in a risk of

fatality that would correspond to the general risk of fatality in fire.

Clearly the result is a function of the architectural layout of the building

considered and hence the estimation of the ratio of possible deaths to the

total number of occupants of the building.

It is shown that the risk of fatality due to progressive collapse per

year could be written

y- X No of units x 16

For a given architectural layout, and given values of N and Y, the risk

appears to be directly proportional to the number of units, namely the number

of stories.

The use of the figure of 433 incidents should be qualified to account

for the assumptions made and for its dependence upon architectural layout

and size of building. Nonetheless, the figure of 433 incidents establishes

an order of magnitude that is useful in assessing the significance of the

statistics for abnormal loadings that are discussed in Chapter 5 and summarized

in Chapter 6. In Chapter 6 it will be shown that a lower bound estimate of the

number of abnormal loadings per year on housing units is 702.

435
IV. Classifications of Abnormal Loadings

Publications such as the Engineering News Record regularly document

engineering failures as well as successes. The authoritative work of Feld

[12] has described many building failures. Allen and Schriever [7] have

summarized reported failures of recent years in North America. Two conclusions are drawn from works such as these: Only a small minority of building

failures occur due to a loading of a type explicitly considered in the design.

The great majority of failures result from loading conditions to which current

codes and standards give little or no guidance and which, as a consequence,

are not considered in design. Such loading conditions are termed abnormal

loadings in this report. The second conclusion is that there is a large

variety of abnormal loadings and no classification of them could reasonably

be expected to be complete. One important contributing factor is that, with

ever advancing technology, new sources of abnormal loadings can be expected

to be generated. With these several thoughts in mind, the following classifications are deliberately limited to abnormal loadings for which the probability

of occurrence seems significant. .The first classification is an overall

generic one:

violent change in air pressure

accidental impact

faulty practice

f oundati on fai 1 ure

The subc 1 as s i f i cati ons will now be discussed in more detail.

A . Violent Change in Air Pressure

These include:

Sabotage bombings

Service system explosions

Other explosions within the building

Explosions external to the building

Sabotage using explosives is a very serious form of abnormal loading.

The motive for sabotage might concern only one person, a family, or an organization resident in the building, yet the bombing could affect many or

all of the occupants. Service system explosions can originate in heating,

cooling, and cooking systems, in high-pressure steam pipes and in boilers.

Sources of other internal explosions include containers of liquified gases

such as propane or butane or containers of gasoline. There are a number of sources of accidental explosion external to the building such as the shipment

of hazardous materials through urban areas by truck, railroad, and waterway

or by the rupture of gas transmission and distribution systems.

B . Accidental Impact

These i ncl ude : Highway Vehicles

Construction Equipment

Ai rcraf

t

Trucks and automobiles leaving the highway out of control are included

in the first category. Accidents involving cranes and lifting devices of

all kinds are included in the second category. In urban areas, construction

436
frequently takes place on congested sites that have relatively small clearances from existing occupied buildings.

C. Faulty Practice

Past experience would indicate that when failures do occur, they are frequently the result of faulty practice. Whether or not local or extensive

collapse results, is largely a function of the type of construction involved,

i.e. whether it can tolerate local damage without extensive collapse.

Design Error

Construction Error

Misuse or Abuse by the Occupant

Misuse or abuse by the occupant can include ill-considered architectural

changes or cutting of the structure.

D . Foundation Failure

The ASCE Research Council on Expansive Soils has documented [13] that

building foundation failures and distress account for average annual property

damage in the USA valued at $740 million. While this figure is not broken

down into specific categories of failure, it is nonetheless indicative that

present codes and standards may not provide adequate requirements for foundation design. Feld [12] has documented a number of instances in which foundation failure has produced severe building distress and even total collapse.

It is apparent that foundation failures, including the following specific

cases, can pose severe abnormal loadings:

Unforeseen Settlement

Foundation Wall Failure

Scouring Action of Floods on Foundations

Adjacent Excavation

An important factor is the growing scarcity of land in urban areas causing more and more buildings to be located on sites previously considered

to be of marginal quality for construction purposes.

V. Studies of Abnormal Loadings

The NBS studies to date have revealed that statistics are compiled by

appropriate authorities with respect to sabotage bombings, gas explosions,

explosions of hazardous materials in transit, highway vehicle accidents and

aircraft accidents. Efforts to locate data regarding other abnormal loadings

will continue, however, it is recognized that, for certain incidents, statistics may either not be compiled or they may be collected in such a fragmented manner as to make them of little value.

A . Sabotage Bombings

Sabotage bombings are generally classified as explosive or incendiary

but it is the former that are of particular relevance to progressive collapse.

A.l Characteristics of Loading

There is no shortage of published technical information relating to the

pressures, rise time, and distribution relationships for explosive charges.

Organizations such as the U.S. Department of the Army have developed and

437
distributed materials that serves as a guide in the use of explosives in the

destruction of military obstacles and in certain construction projects. The

Army Field Manual [14] provides information including type, characteristics,

and uses of explosives and auxiliary equipment, preparation, placement and

firing of charges, and charge calculation formulas.

Figure 5 due to Granstrom [15] expresses pressure-time curves at

selected distances from a 1 kg (2.2 lb) charge of T.N.T. The ordinates

express the static peak pressure in atmospheres. As an illustration, at a distance of 1 meter (39.4 in) from the detonating charge the peak static

pressure would be approximately 11 atmospheres corresponding to approximately

160 psi . This positive pressure pulse would have a duration significantly

less than 1 millisecond. A pressure of 160 psi is so large in comparison

with the normal resistance of walls and floors as to make their destruction

a certainty. At a distance of 10 meters (32.8 ft) from the charge the peak

static pressure would be approximately 1.5 psi while the positive pressure

pulse would last approximately 4 milliseconds. Pulses of these durations

are so short, in comparison to the natural period of building elements such

as walls and floors (20-40 milliseconds) as to require any structural analysis

of the element in question to be a dynamic one.

The characteristics of explosive charges differ considerably from those

of flammable gases. According to Rasbash [16].

"As a rule, gas and vapor explosions take place substantially more

slowly than explosions involving high explosives such as T.N.T. The most

explosive mixture of a fuel vapor and air in a volume of 30m^ (1050 ft3) will

contain about 2.5 kg (5.5 lb) of fuel. The energy potential of this will be

equivalent to that of 20 kg (44 lb) of T.N.T., but the pressure pulse with

the gas explosion would last several hundred milliseconds and with T.N.T.

only about 1 millisecond." For comparison, Figure 6 is included to show the

pressure-time curves for typical vented gas explosions.

A. 2 Probability of Explosive Bombings

Two organizations have gathered national statistics for sabotage bombing

in the USA, namely the Federal Bureau of Investigation and the International

Association of Chiefs of Police.

The Federal Bureau of Investigation (FBI) commenced its program to

collect and classify bombing incidents at the start of 1972. The Bureau

issues monthly bulletins [17] summarizing the data reported by its Field

Agency; it is understood that the first annual report, containing statistics

for 1972, will be issued in the spring of 1973. Figures for the 10 month

period, January through October 1972, are given in Table 5.1. The term

"actual" is used to denote that detonation of an explosive or ignition of an incendiary material actually occurred, whereas "attempted" denotes that

detonation or ignition of the bomb did not take place.

Because reports continue to trickle in long after the reporting period

has passed, the FBI cautions the user of the data that the figures are subject

to revision (in an overall upward sense). The figures of the most recent

months are likely to change most. The monthly totals should therefore not be

used in an attempt to define trends.

Referring firstly to Lines 1 through 5 of Table 5.1, it is seen that,

during the 10 month period, there was a total of 608 actual explosive bombings out of a total of 1689 incidents of all categories. The ratio of

actual explosive bombings to actual and attempted, explosive and incendiary

incidents is 608/1689, namely 0.36.

The FBI uses a number of categories with which to describe the target.

438
r = 0 2m

r=0 5m

r=1 Om

' J —J j_ ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

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Figure 5 Pressure-time curves at selected distances (r)

from a 2.2 lb (1 kg) charge of TNT. The

ordinates give the pressures in atmospheres

above atmospheric pressure. The horizontal

dashed lines correspond to a total vacuum,

g

= gas, a = air. (Due to Granstrom, reference

1.5, reproduced from [70]).

439
Figure 6 Pressure-time curves for typical vented gas

explosions. Py = pressure at which vents fail.

Broken line A denotes the subsequent pressure

rise in the absence of vents. Upper curve shows

the effect of smaller vents. Time scale is only

approximate. (Due to Mainstone, reference 70).

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For brevity only four main categories are given; those having the most

bearing on the subject in hand. Lines 6 through 9 show figures respectively

for the main categories: commercial buildings, office buildings, automobiles

and residences. It is important to note that the listed category of target

is that in which the incident took place. Thus, if an office unit within a residential building were the target of an incident, the occurrence would be

listed under category "Office Building" not "Residences." If the unit had

been a store rather than an office, the entry would be under "Commercial

Building." Similarly, if an automobile parked in a basement garage to the

residential building was the object of an attack, the incident would be

listed under "Automobiles." This suggests that inasmuch as they could con- stitute abnormal loads for the building as a whole, certain incidents listed

on Lines 7 through 8 might contribute to the figures defining the probability

of explosive loading on buildings i.e., line 9 might constitute a lower bound

to the frequency of abnormal loadings. At this time however, the readily

available FBI data do not permit these incidents to be identified, and for

this reason, only the figures on line 9 will be used further.

The FBI uses the term "Residences" to cover private residence, apartment

house and other private property such as sheds, and garages adjoining the

residential building or in its vicinity. The figures for other private property are not included in the table, however the 10-month total for this

subcategory is 26.

At the present time, the FBI makes no attempt to report the number of

stories in the building subjected to the attack, or the severity of the

attack in terms of structural damage to the building. Such information would

greatly add to the value of the statistics, at least as far as building safety

is concerned. It would also be valuable to have a clear definition of the

difference between a private residence and an apartment house with respect to

the FBI's classification of incidents.

Probably the most useful figure to use, in order to assess the frequency

of abnormal loadings due to sabotage bombing, is the 10-month total for

residences, 471. To arrive at an estimate of the number that were actual

explosive incidents this number is multiplied by 0.36 giving 170. If this

is converted to a yearly estimate the result is 204. It appears therefore,

that at the present time, 566 sabotage bombing incidents per year occur

where the direct target is a housing unit. Of these, an estimated 204 can

be expected to involve explosives with which detonation takes place. For reasons discussed earlier, this figure may be a lower bound; it is also

conceivable that not all incidents were recorded by the FBI during this

initial 10-month period of their data collection.

Corroborating evidence to support the FBI data is provided by the ICAP

[18] and, on a State basis, by the California Department of Justice [19].

In the period July 1971 through February 1972, the lACP operated the National

Bomb Data Center under the auspices of the U.S. Department of Justice. Since

that time, the functions of the Center have been transferred to the FBI. The

monthly summary reports of the Center, during the period July 1971 through

February 1972, provide a brief description of each incident in addition to a statistical treatment of all bombings for the month in question. One description, taken from the January 1972 report [20], serves to illustrate the

possible scale of the risk:

"January 5, Las Vegas, Nevada. An explosive device

,

consisting of 40 sticks of dynamite , was placed in the

laundry room of an apartment building . The blasting

cap detonated , but improper assembly of the device , and

the age of the dynamite combined to produce only partial

detonation. No damage resulted."

Had the explosion occurred and had the building been of a design similar to

that used at Ronan Point, the United States may have had its first "Ronan

442
Point" incident, with considerable political and professional repercussions.

B . Gas Explosions

This subsection deals with accidental loadings arising from explosions

occurring accidentally as a result of the distribution and use of gas. Be- cause of Ronan Point, the dynamics of gas explosions and their interaction

with building construction of various forms have received considerable study

in the United Kingdom. The greater part of U.K. gas is still manufactured,

yet a rapidly increasing percentage is of natural origin from North Sea sources. In contrast to the U.K., the U.S. production of manufactured gas

amounts to only a few percent of the total, natural gas providing almost all

the needs .

B.l Time -Magnitude-Distribution of Load

Rasbash documented [16] the maximum fundamental burning velocities

of some gas-air mixtures under atmospheric conditions. Table 5.2 shows

manufactured gas to burn approximately three times as rapidly as natural gas.

Such data has caused certain members of the engineering profession to make

some distinction between the two types of gas when considering the risk of

progressive collapse.

Table 5.2 Fundamental Burning Velocities (Maximum) of Some

Gas-Air Mixtures Under Atmospheric Conditions

Due to Rasbash [16]

Burning Velocity

Gas or Vapor f t/s m/

s

Methane (natural gas) 1.2 0.37

Butane 1.3 0.40

Hexane* 1.3 0.40

Propane 1.5 0 . 46

Ethylene 2.3 0.70

Manufactured Gas** 4.0 1.2

Acetylene 5.8 1.8

Hydrogen 11.0 ' 3.4

* Similar to gasoline.

Approximate value for manufactured gas - depends on composition.

Alexander and Hambly [21] discussed various means of reducing the

consequences of gas explosions in buildings; such as, complete removal of

the explosive source, forced ventilation, and control of the maximum flow

of gas from the supply. They provided a qualitative description of the

443
nature of a gas explosion in terms of gas/air mixture, presence of venting,

pressure rise, turbulence, and other related parameters. They also provided

a method of analysis of structures subjected to these dynamic loads.

In another paper, Alexander and Hamley [22] developed a method for design

of structures to withstand dynamic loading from gaseous explosions similar to

that which caused the collapse at Ronan Point. The loading pressure pulse

was described only qualitatively; a precise description could not be given

at that time due to the absence of relevant experimental data. The response

of the structure to such dynamic loads was discussed and a method of design

presented, with examples for floor, slabs, and load-bearing walls. Proposals

were given for research that should be carried out to determine the design

pressure pulse and to check the validity of the assumptions made.

Stretch [23] examined how explosions, caused by vapor-phase reactions

between common inflammable solvents or sources of energy and air are con- trolled by particular features of domestic buildings, and consequently strain

or damage the structure during their passage. He concluded that the general

use of materials such as gas is a safe and convenient element in contemporary

society, so that, today, homes are considered fit for human occupancy only

if they are designed to withstand and contain, within tolerable limits, the

risks incumbent on a high standard of living. He showed how inherent features

of established structural systems have obscured the necessity of special

precautions in more recent systems. Stretch gave simplified forms of the

pressure waves and used these to analyze the behavior of buildings of traditional brick design, framed construction, and concrete panel construction,

respectively.

Rasbash [15], in a paper accompanying that presented by Stretch [23],

discussed the influence of potential relief of explosion pressures provided

by external windows and doors during gas and vapor explosions. He provided

a quantitative approach for estimating these pressures.

To define experimentally, those data needed in the mathematical modeling

of gas explosions in buildings, Rasbash, Ralmer, Rogowski and Ames [24]

carried out experiments in which they exploded mixtures of air and manufactured gas or natural gas respectively. These explosions took place in a strong chamber with partitions simulating the division of a building into

rooms

.

Further experiments were carried out by Astbury, West, and Hodgkinson

[25] to investigate the effect of different gas layering conditions in a pair

of rooms. During the tests, different layers of both gas and a gas/air

mixture were used. In two of these experiments, the most explosive (stoichiometric) manufactured gas/air mixtures were obtained and the resulting explosion caused, in one case, minor, and in the other case, major damage to the

3 1/2-story building which was of load-bearing brick. These experiments were

a repeat of earlier tests in which an attempt was made to demonstrate the

effects of turbulence. Turbulence has the effect of increasing the pressures

developed when an explosion proceeds from one room to another filled with gas,

namely, the "cascade" effect. The test demonstrated the effectiveness of

venting in limiting the maximum pressure developed in an explosion. Despite

suffering the extensive damage, the brick building could be safely propped

and no progressive collapse occurred.

West [26] carried out tests involving the effect of gas explosions on windows of various details in order to study their effectiveness in providing

venting. Specimens included single-glazed windows of 32 oz glass and doubleglazed units of the same thickness of glass. Because of their higher

strength, the double-glazed windows provided ineffective venting. Further,

the resistance of glass to short-term loads (defined as lasting 3 seconds)

is more than twice that under sustained loadings. Repeated explosions that

do not break the glass may be the cause of eventual failure at a lower

444
pressure. The strength of glass decreases with time. For example, glass

eighteen months old failed at loads some 20 percent less than those obtained

with newly-manufactured glass. Finally, while the failure of glass may give

an indication of the pressure developed in a real incident, care is necessary

in interpreting test results since a distortion of the frame can cause the

glass to break at a pressure less than its actual breaking strength.

Mainstone [27] reviewed the existing experimental data on the strength

of glass under loads of very short duration. He provided the basis for a graphic presentation of likely breaking pressures, under gas-explosion

loading, for particular sizes and thicknesses of window panes. An earlier

review by Rasbash of data on the venting of gas explosions was then used

as the basis for extending the graphic presentation to cover also the

possible rise in pressure after the glass is broken by an explosion. The

graphic presentations can be used directly for estimating the pressure reached

in actual explosions from observations on the damage to glass windows; and

they were prepared primarily to be used in the design of glazing as an explosion vent.

On the basis of a review of the above studies it is concluded that there

is sufficient data available to allow satisfactory prediction of the characteristics of gas explosions in buildings providing the gas mixtures can be

de f i ned .

B.2 Probability and Consequence of Load

The probability of gas explosions has been studied both in the United

States and the United Kingdom. Whereas the Ronan Point incident did not occur until 1968, the American Gas Association, which represents approximately

85 percent of the U.S. gas industry, had compiled statistics of gas incidents

some years earlier. Before reviewing the results of the AGA studies, it will

be useful to consider the studies in the U.K. to gain perspective.

In the Report of the Inquiry into the collapse at Ronan Point, Griffiths,

Pugsley and Saunders [1] assembled the data available at that time dealing

with the probability of gas explosions in the U.K. Table 5,3 is taken from

the report of Griffiths et al and contains an analysis of explosions in

•housing for each of the years 1 957 through 1966.

Structural damage is defined as damage to the structure over and above

the mere blowing out of windows and window frames. It will be seen from Tables

5.3 and 5.4 that, of the known causes of explosions, manufactured gas is the

principal hazard. In the year 1966 there were approximately 18 million

housing units (apartments and houses) in the United Kindgom and, of these,

approximately 12,260,000 were supplied with manufactured gas. The 1966

figures show that the frequency of explosions involving manufactured gas in

premises supplied with gas is approximately 8 per million dwellings, of which

I 3.5 per million will be of sufficient violence to cause structural damage.

' Griffiths et al assessed the chance of a gas explosion in a high apartment

I building. In a building the size of Rcnan Point, with 110 apartments and a

I life of 60 years, there is slightly more than a 2 percent risk that a gas

j explosion causing structural damage will occur in one of the apartments during

the lifetime of the building, i.e., 3.5 x 10-^ x 110 x 60 x 100 = 2.31 percent.

I In other words, the chances are that of every 50 such buildings, one will

I experience structural damage as a result of a qas explosion in its lifetime.

They pointed out that, whereas it may be argued that it is cheaper to prohibit

I the use of gas in tall apartment buildings than to make the structures free

I from the risk of progressive collapse, they did not accept this argument.

' They reasoned that gas is justifiably regarded as a safe and acceptable fuel

j in domestic premises generally [28]. Furthermore, the banning of gas would

not, of course, completely eliminate the risk of damage to the structure of a tall apartment building, resulting in progressive collapse, although admittedly

it would remove the most likely cause (sic). There remained the possibility

445

493-610 O - 73 - 30
Table 5.3

Frequencies of Explosions in Housing Units

of Fire Department Reports in the United

Explosive Material Reproduced

Year

iytttiiff1 1 sig Yot-al Manufactured Gas Liquefied Petroleum

Gases

Liquids

Other

anil factor explosive Total Superficial Siriiclural Total Superficial Structural Total Superficial StrucKual unkmnui

1966 l/I 213 97 55 42 14 6 33 25 8 69

Oj 1/1 1 SI 76 40 36 14 8 6 Zrf 20 9 62

64 1/2 168 80 28 52 8 8 18 16 2 62

63 1/6 216 84 54 30 6 6 18 12 6 108

62 1/2 234 70 20 50 8 2 6 34 28 6 122

61 1/2 198 46 28 18 10 4 6 38 30 8 104

60 1/4 144 72 36 36 16 16 12 8 4 44

59 1/4 148 88 24 64 24 20 4 36

58 1/4 192 64 28 36 12 12 48 32 16 68

57 1/1 195 70 41 29 8 3 5 44 35 9 73

Total 1,889 747 354 3'»3 96 25 71 298 226 72 748

Estimated from Samples

Kingdom - Damage and

from [ 1 ]

446
Table 5.4

Explosions in Domestic Premises in the United Kingdom

Reproduced from [1]

Total

Explosions Explosive Material Damage Fault

213

|ylanuf actured gas 97

Superficial 55 Installation 35 User 20

Unknown 0

Structural 42

Installation 26 User 9 Unknown 7

L. P.G. 14 (Liquefied petroleurn

gases)

Superficial 8 Installation 3 User 4 Unknown 1

Structural 6 Installation 5 User 0 Unknown 1

Liquids 33

Superficial 25 Installation 6 User 17 Unknown 2

Structural 8 Installation 0 User 7 Unknown 1 Other and

Jnknown

447
of explosions caused by substances other than manufactured gas, for example,

gasoline or other volatile inflammable liquids, butane gas cylinders,

electrical apparatus, and so on; as well as other forms of accidental damage

(sic).

Prompted by the Ronan Point incident, the U.K. Construction Industry

Research and Information Association (CIRIA) established a pilot survey [29]

to establish the procedures for future and wider surveys on the frequency of

gas explosions and the structural damage they can cause. Newspapers were used

to obtain reports of explosions in residences and, where appropriate, visits were made to damaged properties and comprehensive investigation of the circumstances carried out. Findings showed that gaseous explosions in U.K.

housing causing significant structural damage, occur at the rate of less

than one pe r week

.

Fry [30] examined U.K. fire incidents involving explosions of manufactured gas in dwellings during the 13 years, 1957 to 1969. The average annual

incidence was shown to be approximately 90 but appeared to be increasing as the consumption of gas increases. The average rate of incidents is about 5.0

per 108 therms of gas sold. Approximately 48 percent of the incidents cause

some structural damage and in 40 percent of these it was considered "severe."

From reports involving manufactured gas and natural gas in 1969, it appeared

that natural gas was more likely to cause explosions but that the explosions

were of similar violence for the two types of gas.

A later report [31] of the field survey of damage, caused by gaseous

explosions in the U.K. contained the conclusion that roughly one severe or very severe explosion occurs every two weeks. Furthermore, there is some evidence that the frequency is increasing.

It is now useful to discuss the probability of gas explosions within

the Continental USA. The two principal sources of information on the gas

industry are the American Gas Association and the Office of Pipeline Safety

(OPS) of the U.S. Department of Transportation. The following is an attempt

to evaluate information from each of these sources. Within the Continental USA there are 915,000 miles of gas pipeline [32].

This total is composed of 288,000 miles of pipeline in gathering and transmission systems and 627,000 miles of pipeline involved in subsequent distribution of gas [33]. Since the former are located largely in rural areas, it

is with gas distribution systems that this study is primarily concerned. It

is significant that, within the gas distribution system, gas leaks are reported

to occur at an annual rate of 1 per .1.1 mile of gas line; e.g., a total of more than 560,000 leaks in 1972. More than 300,000 of these leaks occurred

as a consequence of normal wear and tear; i.e,, corrosion, fatigue, material

failure, etc. Leaks are reported [33] to occur with comparable frequency in

both the mains and service lines and about two-thirds of all leaks were associated with the pipeline itself, the remainder being associated with the

fittings and other attachments.

Including both single and multiple dwelling units, almost 34.7 x 10^

housing units in the USA used gas for house heating in 1970 [32]. It is

also estimated [5] that natural gas serves about 55 percent of all housing

units as a fuel for residential space heating and it is significant that of

all new customers for house heating in 1970, 37 percent of them were conversions

to a gas service system. Because a single customer or meter or furnace may

involve more than one dwelling unit, these figures may not be fully representative of the total number of dwelling units to which gas is supplied. The AGA

estimates that, including appliance usage, natural gas is supplied to upwards

of 60 percent of the total residential market.

A prime source of information on gas leak incidents is the summary to the

research report "Public Safety and Gas Distribution" [34] prepared for the

448
American Gas Association by Arthur D. Little, Inc., and dated December 1967.

This was a survey of the gas distribution industry in which 140 companies and

systems participated, representing 83.3 percent of all gas distribution meters

The yearly averages for gas-related incidents are based on the 10-year period

1957-66, As far as incidents involving payment of compensation are concerned,

the yearly averages are representative of the 7 year period 1957-1963. Both

in terms of the time periods and the number of companies involved, this survey

constitutes the most comprehensive, if not the only, study of the overall

safety of the gas distribution system within the continental U.S.

With reference to Table 5.5, assembled from information contained in

the A. D. Little, Inc., summary, it is

be expected in an average year and, of

pensation. A total of 151 (60 percent

of more than $1,000. It is recognized

sions could have been severe enough to

nately, there is no record of:

evident that 1,508 gas explosions can these, 329 will require monetary comof 253) explosions will involve payments

that only a proportion of these explocause structural damage, but unfortu1. The relative proportion of personal and property damage.

Whether or not one or more

the type of building, and

buildings were involved and, if so

The relative

greater than

severi ty

$1 ,000 . of each of the incidents involving payment

It may be presumed that of the explosions incurring payment of less than

$1,000, very few, if any, were likely to have involved significant structural

damage. In order to evaluate a probability based upon the AGA statistics,

it will be assumed that only one building was involved in each of the 151

incidents and that each suffered significant structural damage

tion can be seriously questioned as the following illustration

report [35] would indicate but in the absence of more specific

least provides the basis of a conservative estimate:

This assumpfrom an OPS

data, it at

"Mobile Oil Corporation High Pressure Natural Gas Pipeline

Houston, Texas, September 9, 1969 - Synopsis

At 3:40 p.m. on September 9, 1969, the 14-in pipeline

carrying natural gas at a pressure of more than 780 psig

ruptured in a newly constructed residential subdivision

3 1/4 miles north of Houston, Texas. The escaping gas

created a dust storm like condition and sounded like a jet

engine. Electrical and telephone utility servicemen working

in the area, with the help of local residents, immediately

commenced to evacuate all residents in the vicinity of the

rupture. About 8 or 10 minutes later, the escaping gas

exploded violently. Thirteen houses, ranging from twentyfour ft to 250 ft from the rupture were destroyed by the

blast. The leaking gas caught fire and continued to burn

to a height of 125 ft for 1-1/2 hours until valves on the

other side of the leak were closed by Mobil workmen

dispatched to the valve locations. The fire abated at

that time, but some gas burned for another five hours.

In all, 106 houses were damaged and property damage was estimated at $500,000. Miraculously, there were no deaths but nine people were injured, two seriously."

Office of Pipeline Safety figures for 1971 [35] indicate that, of the

total number of reported explosions, 50 percent occurred in residential

449
Table 5.5

Gas Related Incidents. Their Nature, Annual Incidence and Consequence

(Taken from A. D. Little Summary Report to American Gas Association [34]

10 YEAR AVERAGE

"1

7 YEAR AVERAGE - 1957-63 INCLUSIVE

NATURE OF INCIDENT

INCIDENT

REPORTED

INCIDENTS

INVOLVING

PAYMENT*

SIZE OF PAYMENT

>$1,000 \ <$1,000

>$ 25.

NUMBER % NL^MBER % % %

EXPLOSION 1508 12.8 329 13.3 60 10 4

FIRE 1835 15.6 274 11.1 36 9 7

PRODUCTS OF COMBUSTION 1228 10.4 ]G9 4.4 7 5 3

UNION ITED GAS 3118 26.5 1498 60.7 12 66 69

FLASHBACK 4122 35.1 217 8.8 6 9 11

OTHER 344 2.9 167 6.8 3 7 8

103.3"^ 105.1"*" 124"^ 106"^ 102"*"

TOTAL NUMBER

OF INCIDENTS 11753 2466

253 606X^1

y 24.6,;d

* Total Payments by Gas Company exclusive of Company repair costs.

Some incidents may involve more than one of the listed phenomena.

450
buildings and 7 percent in commercial buildings. The remainder occurred in

manholes, regulator pits, etc., and it is unlikely that these involved

compensation in excess of $1,000 since, in most instances, these incidents

occurred on or within gas company property, it is unlikely that they involved

the gas company in claims for payment. Accordingly, it is presumed that 50/

57, namely 87.5 percent of the 151 incidents, namely 131, were likely to have

involved residential buildings.

The period over which the AGA statistics were gathered centers approximately on 1960. According to the 1960 Census of Housing [36], the occupied

housing in the U.S. totaled 58,314,784 units. Accordingly, a crude estimate

of the annual probability of occurrence of a structurally significant gasrelated explosion in a residential unit is of the order of 131/58,314,784 x 100 = .00022 percent or 2.2 structurally significant explosions per million

dwelling units per year.

During the period 1957-1963, the average annual natural gas sales to

residential customers totaled 300 x 10° therms. In terms of energy, the

annual rate of structurally significant gas-related explosions is 0.44 for

every 108 therms of gas supplied. If all gas-related explosions and the

total amount of natural gas sales are considered, the probability of explosion is 1.67 explosions per 10^ therms per year.

In accordance with the provisions of the National Gas Pipeline Safety

Act of 1968 [33], a nationwide reporting system for gas-related incidents was initiated in 1970 by the Office of Pipeline Safety (OPS). Detailed reports

are required of individual incidents that involved one or more of the following criteria:

1. Caused a death or a personal injury requiring hospitalization;

2. Required any segment of transmission pipeline to be taken out of

service;

3. Resulted in gas igniting;

4. Caused estimated damage to the property of the operator, or others, or both, of a total of $5,000 or more;

5. Required immediate repair and other emergency action such as evacuation of a building, blocking off an area, rerouting of

traffic to protect the public; and

6. Were deemed significant but did not meet the criteria of 2, 3 or 4.

All gas companies with more than 100,000 customers (over 85% of the

total number of gas customers) are required to submit reports within 30 days

of the incident. Records for 11 months of T970 and all of 1971 are publicly

avai 1 abl e [33] .

In an NBS Technical Note entitled "Residential Buildings and Gas-Related

Explosions" [37] the AGA and OPS statistics are fully discussed. It is shown

that at this point in time the AGA and OPS statistics are in conflict and of

the two the AGA statistics would appear to be the more representative since:

1. They attempt to cover all incidents, both upstream and downstream

of the meter in distribution systems (see Table 5.6),

2. They represent a 10-year average whereas the first complete year of

OPS statistics is 1971.

In that report, it is concluded that, though due regard has to be taken

451
Table 5.6

Relative Location of the Basic Cause of the VarioiK. r^^ p«i.^ a

10 YEAR AVERAGE 7 YEAR AVERAGE

LOCATION OF BASIC

CAUSE OF INCIDENT

TOTAL NUMBER OF

INCIDENTS

TOTAL NUMBER OF

INCIDENTS

INVOLVING PAYMENT

NUMBER % NUMBER %

BEFORE OR AT THE METER

I.E., UPSTREAM

3,122 26.5 1,780 ' 72.1

AFTER THE METER

I.E., DOWNSTREAM

8,296 70.6 639 25.9

OTHER 335 2.9 47 2.0

TOTAL NUMBER OF

INCIDENTS PER YEAR

11,753 10 'i'lo 2,466

452
for the limitations inherent in the available statistics, the probabilities

of the occurrence:

1. Of a gas explosion capable of causing significant structural damage

is 2.2 per million housing units per year.

2. Of a gas explosion capable of causing significant structural damage

in a hundred unit apartment building during a 50-year service life

is 0.011 or 1.1 percent.

C . Explosions of Hazardous Materials

The National Bureau of Standards is presently gathering statistics of

explosive incidents arising from the normal transportation of hazardous

materials in urban areas by road, rail, and waterway. Such materials include

petroleum and its products, chemicals, explosives, and liquified gases.

Approximately 20 percent of all hazardous materials transported in the USA

are moved on waterway [38]. Waterway explosions of such a nature that

buildings are damaged in the course of the incident are rare, yet the potential for such an incident is not only present but increasing, particularly

as the tonnage of petroleum and petrochemicals shipped by water increases.

Another source of potentially extreme loading exists where fuel transportation

terminals are located. Such terminals provide a storage facility for large

volumes of petroleum which have been transported on waterways. Storage tanks

are now located in many densely populated areas including Providence, Boston,

Staten Island, and Philadelphia; more are planned.

It would appear that the shipment of hazardous materials, by road and

rail, poses a problem that is not negligible. The explosion of unconfined

vapor clouds, produced by the dispersion of flammable liquid or vapor spills,

is becoming a serious problem contends Strehlow [39]. He points out that

this is mainly because of the increased size of the spills in recent years.

Illustrating this point is the accident that occurred in East St. Louis,

Missouri on January 22, 1972. Since the National Transportation Safety

Board is currently investigating the incident, and the report is not yet

released, the following information was obtained from a file of newspaper

clippings maintained by the Safety Board; it should not be considered

of f i ci al :

The accident occurred in an East St. Louis railroad yard where

a process known as "humping" was taking place. In this process

,

a railroad car is allowed to roll freely toward other coupled

cars with sufficient momentum to allow the couplers to fasten.

This is a process regularly used to make up trains. Alledgedly,

in this instance not 1 but 4 already coupled cars were being

humped. The lead car, filled with 500,000 lb (30,000 gallon)

of propy lene , a derivative of liquid petroleum gas (LPG) , was travelling at approximately 15 mph , instead of the recommended

6-7 mph, when it bumped a hopper car at the end of the partlyassembled train. The total momentum of the impact was such that

the hopper car coupler jumped over the coupler of the lead rail car and punctured its propylene tank. The cars continued to move for several hundred feet. The released propy lene formed

an unconfined vapor cloud which exploded 500 ft. from the location

of the leaking car.

The explosion shook a 4 square mile area and shattered windows 8 miles away; the concussion was felt up to 20 miles from the scene

of the accident. The explosion caused roofs and walls to collapse

as much as 6 blocks away. At least 3 separate fires followed the

explosion and involved 30 additional railroad cars. Apparently

none of these additional cars contained hazardous materials , for

453
no further explosions were reported

.

Early reports estimated that 1,000 buildings had been damaged

including 650 homes and 350 business buildings. Later information

indicated that 868 buildings had been reported as damaged . One

hundred families were left homeless. In all 176 people were injured and total non-railroad property damage was estimated to

be 6.5 million dollars

.

It is estimated by the National Transportation Safety Board that the

report will be completed by January 1973. Strehlow sites a 1962 accident in

the State of New York that involved a truck:

July 26, 1962 New Berlin, New York [40]. 7,000 gallon tank

truck in a single truck accident . Tank ruptured catas trophi cal ly

in town. Vapor cloud covered 200,000 square feet and was 80 ft

deep before ignition . Explosion and following fire killed 10,

caused $200,000 damage.

Strehlow states that the characteristics of the initial fire or explosion,

which follows the ignition of a spill, depend on four things:

1 ) The nature of the fuel

.

2) The rapidity of the spill coupled with the wind conditions,

terrain and/or location of nearby buildings.

3) The delay before an ignition source is found.

4) The nature of the ignition source.

He concludes that current theoretical results are unable to predict the

observed pressure-time behaviors.

Strehlow [39] has tabulated 108 accidental unconfined vapor cloud

explosions that have been documented over the past 42 years. The list is

not complete because it is limited almost entirely to explosions that have

occurred in the USA and Germany, because the documentation of individual

explosions has often been fragmentary, and because information about many

on-site plant explosions are not accessible to the general public. Using

Strehlow's data, Figure 7 has been plotted showing the average number of

explosions per year over periods of 2, 5 or 10 years. It is seen that the

number per year is increasing rapidly. In approximately two years, 1970

through January 1972, a total of 15 incidents occurred, causing damage estimated at 23-27 million dollars. It appears from Strehlow's paper that, with

one exception, these occurred in the United States.

How many housing units were subjected to severe blast loading is not

known with any certainty, but it is clear from the incident at East St. Louis,

January 22, 1972, described earlier, that the number would be a significant

one. The reports of the incident, which has been previously discussed, estimated that 650 homes were damaged and 100 families were left homeless. It would appear that a lower bound to the number of housing units subjected to

abnormal loading due to vapor cloud explosion was 100. The non-railroad loss was 6.5 million dollars, out of an estimated total of 23-27 million dollars

for the last two year period shown in Figure 7. If it is assumed that the

number of housing units affected in two years can be arrived at by prorating,

the figures for St. Louis then this number would be approximately

23 100 X ^—F- =^ 354 housing units

454
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455
On this basis, albeit a crude one, it could be expected that 354/2,

namely 177 incidents occur per year unless some change in the incidence of

accidental unconfined vapor clouds were to take place.

C.l Highway Vehicle Impact

Sanders [41] has developed relationships to aid the structural designer

in estimating the probability of a structural component being struck by an errant motor vehicle. An example of this type of incident would be a motor

vehicle straying from its normal path and hitting a building column. The

probability of failure of the column is derived from combining the probabilities: (a) of a vehicle striking the column and (b) that the vehicle has the

mass, velocity, and stiffness to cause the failure of the column. The probability of a motor vehicle striking an object located near a traffic path is

derived as a function of (a) the distance of the object from the traffic path

and (b) the volume of traffic flow.

For the USA in 1970, the total number of motor vehicle accidents of all

types was approximately 16 million [71]. Highway accident statistics are compiled by each State and the categories of information on the reporting

forms frequently total as many as 60 items. In spite of this, incidents in

which vehicles collide with buildings receive scant coverage. Indeed, it is

the exception for the report to identify the object struck when a vehicle

leaves the highway. The U.S. Department of Transportation has assisted NBS

in identifying two states in which building strikes were considered in the

data collection. These States are Oklahoma and Illinois, the former having

relatively little urban area while the second has large urban concentrations.

In each case the data analyzed were those for 1970.

Throughout the year 1970, there were 65,183 motor vehicle accidents in

Oklahoma. In 50 of these a vehicle was reported to have collided with a building. In Illinois in 1970, there were 409,174 motor vehicle accidents

and of these, the number in which a vehicle was reported to have collided

with a building, is 1229.

If the following assumptions are made: 1) the Oklahoma data, added to

the Illinois data, is representative of the USA as a whole; 2) the number of

vehicle collisions with buildings (C) is proportional to the population (P),

then based on 1970 data:

'US

'us

'US

'US

ILL

ILL

1279

'OKLA

OKLA

13,674,000

19,000 (approximately)

203 ,21 2 ,000

This estimate is for the number of vehicle collisions with buildings

for the USA as a whole in 1970, based upon combined Oklahoma and Illinois data

It is interesting to differentiate between urban and rural situations,

where urban is used to denote an incorporated area having a population of

2500 or more

.

URBAN

:

'US

1 171

X 149,325,000

1 0 ,970 ,000

16,000 (approximately)

456
RURAL

:

C US 53 ,887 ,000

2 ,704,000 ^ -^0,00

3000 (approximately)

On this basis, it is estimated that, nationally, the 1970 figure for

vehicle-building collisions is 16,000 in urban areas and 3,000 in rural areas .

So far the discussion has dealt with reported collisions of vehicles

with buildings, with no consideration for what is meant by 1) vehicle collision, and 2) building. Vehicles vary widely in mass, and collisions can range

from a gentle nudge to an impact capable of removing a structural element.

The term building covers a range of structures from barns to multistory

apartment houses. As far as this study is concerned, all standard reporting

forms are unsatisfactory in these respects. Efforts to break out the particularly-relevant data are continuing; however, it seems unlikely that the

probability of abnormal loading of housing units due to highway vehicle

impact will be defined with any clarity unless: 1) existing data recording

procedures used by the States are slightly modified, 2) a separate detailed

study is made (this could be based on statistical sampling and involve relatively small areas of the country).

However, with due regard to the uncertainties inherent in the data

available, a lower bound estimate can be made of the number of abnormal loads

per year on residential structures subjected to highway vehicle impact. From

a study of what detailed information is available, it is presumed that at the

national level at least one tenth of the incidents will involve residences

and, of these, one tenth will involve impact sufficiently large to be con- sidered an abnormal loading.

On this basis a lower bound estimate of the number of abnormal loadings

on residential buildings per year due to highway vehicle impact is 190 or,

on the average, four per State.

C.2 Aircraft Collisions With Buildings

In comparison with other forms of abnormal loading the probability of

such incidents is trivial. Using data furnished by the U.S. Federal Aviation

Authority, the NBS study has shown that for buildings located further than 3 miles from the end of an airport runway^the probability of collision with

buildings per year is approximately 10" .

Chapter 5 discussed the available statistics from which preliminary

estimates have been made of the annual incidence of five sources of abnormal

loading on residential buildings. Reference to Chapter 4 will show several

abnormal loadings for which data has not been presented. Efforts to quantify

the probability of occurrence of these other loadings will continue; however,

it should be recognized that for several reasons this quantification may never be satisfactorily accomplished.

In arriving at figures in Chapter 5, it has been the author's intention

to err on the low side in cases of doubt about the available data. For this reason and because not all sources of loading are included, it is believed

that the following summation provides a lower bound result.

VI. Summary of the Probabilities of Abnormal Loadings

And Their Implications for the USA

457
Source

Number of Abnormal

Loadings Per Year

Explosive Bombing 204

Gas Explosion 131

Explosion of Hazardous

Materials 177

Highway Vehicle Impact 190

Summation 702

Chapter 4 discussed the national risk due to fire as a basis for

assessing the incidence of abnormal loadings in housing units. It was shown

that architectural layout and building size affected the number of abnormal

loadings on housing units per year that, in buildings susceptible to progressive collapse, would pose the same risk as fire. For a 100 unit building,

with 4 apartments per story, an average of 4 persons per apartment, and a central service core, this number correspond was 433, while for a 50 unit

building of similar arrangement the corresponding number would be 866.

Comparing these figures with the lower bound estimate of 702 abnormal

loadings a year, it appears that, for buildings susceptible to progressive

collapse, abnormal loadings do pose a risk comparable to that of fire. In the case of the fire risk, specific design criteria are currently implemented.

The United Kingdom Authorities introduced criteria to minimize the risk

of progressive collapse upon concluding that the frequency of explosions,

involving gas in housing units supplied with gas, was approximately 8 per

million housing units per year, of which, 3.5 per million were of sufficient

violence to cause structural damage. It should be noted that this figure

relates to only one source of abnormal loading.

In the USA, the frequency of abnormal loadings on housing units per year

appears to be in excess of 702/(67.7 x 10°), namely approximately 10 per

million. This data appears to provide adequate justification and need for

standards-writing bodies to adopt criteria explicitly to deal with abnormal

loadings and progressive collapse.

VII. Alternative Approaches for Criteria

Each of the following approaches, or some combination, may be used in the

preparation of criteria to reduce the level of risk from abnormal loadings:

Eliminate Source

Reduce Magnitude

Limit Extent of Structural Damage

Resist Local Structural Damage

The banning of gas from buildings would illustrate an attempt to

partially implement the first of these approaches. The second is illustrated

by regulations that might call for buildings to be sited at distances from a highway and with such barriers that impact from vehicles becomes unlikely.

The provision of alternate paths for loads in the event of the loss of a 458
critical member illustrates the third approach. The fourth approach is the

basis of the U.K. criterion in which the element is designed to resist 5 psi

pressure in any direction, applied to the element and to those elements

attached to it.

For the most part, the first two approaches are non-structural.

A . Non-Structural Design Criteria

It could also be feasible to consider introducing criteria in non-structural areas including the following:

Zoning, Siting, Planning

Regulations for Service Systems

Transportation Regulations

A.l Zoning, Siting, and Planning Criteria

The following are examples of areas in which criteria could be considered

for development or modification:

1. The location of gas mains in urban areas.

2. The location of multistory buildings with respect to highways,

railroads, and waterways.

3. The location of multistory buildings with respect to similar

buildings.

4. The zoning of land for residential use, taking into account the

possibility of local external explosions.

A. 2 Regulations for Service Systems

The following are features of service systems that might be studied with

a view to reducing risk:

1. The location of high pressure gas riser mains in multistory

buildings.

2. The location of boiler rooms and furnaces and HVAC plant in general.

3. Ventilation provisions for gas mains, boilers, and furnaces.

4. Specifications for pipe work and fittings.

5. Maintenance and inspection procedures for plumbing installations.

As an illustration, French regulations [42] prohibit the use of gas in

buildings higher than 50 meters (164 feet), and this law was enforced before

the Ronan Point collapse. Furthermore, when gas is installed in buildings,

French codes have requirements for ventilation that are more stringent than

most codes; e.g., the gas supply pipes are enclosed in a ventilated duct

space

.

A. 3 Transportation Regulations

Considerable volumes of hazardous materials are transported through

459
urban areas in the U.S. by means of truck, railcar, and waterway traffic.

Minimization of the risk incurred might come about from new or improved

regulations following a study of:

1. The type and volume of hazardous materials transported as one cargo.

2. The engineering specifications used in the regulation of the design

and maintenance of vehicles used in this transportation, the standard

procedures for the operation of these vehicles, and the procedures

for the supervision of operatives.

3. The routes over which this transportation is permitted to take place

and, in particular, the proximity to urban areas.

4. The statistics for explosions of hazardous cargoes in shipment.

VIII. Philosophies for Structural Criteria

Criteria to minimize progressive collapse are being implemented in

several countries including the Nordic Countries, Countries of Eastern

Europe, France, Italy, United Kingdom, the United States and Canada. A

detailed review of the contents of these various criteria is beyond the

scope of this paper. It is sufficient and more appropriate to identify

the various philosophies that have been used in the preparation of these

criteria:

1. The Cautionary Note : Whereby the attention of the structural

engineer is drawn to the risk of progressive collapse and he

is urged to take precautionary measures to deal with it, as in

the National Building Code of Canada, 1970 Edition [43] which

states that: "Buildings and structural systems shall provide such

structural integrity that the hazards associated with progressive

collapse, due to local failure caused by severe overloads or abnormal loads, not specifically covered in the section, are reduced to a level commensurate with good engineering practice."

The 1972 American National Standards Institute A58, Minimum

Design Loads in Buildings and Other Structures [3], also gives

a cautionary note as follows: " Progressive Collapse - Buildings

and structural systems shall provide such structural integrity

that the hazards associated with progressive collapse such as that due to local failure caused by severe overloads or abnormal

loads not specifically covered herein are reduced to a level

consistent with good engineering practice."

2. The Alternate Path Approach : Criteria of this form call upon

the designer to consider successively, the removal of structural

elements or combinations thereof from the building in a systematic

manner, and insure thereby through analysis and design, that the

building remains capable of withstanding a specified combination

of loads albeit with a small load factor.

3. Specified Abnormal Loadings : In this approach, an equivalent static

loading consisting of a uniform pressure is assumed to envelope the

loading states that might be produced by the various abnormal

loadings on buildings. A structural element or combination of

such elements, that may not be removed in accordance with the

previous approach, is required to withstand the application of

this pressure to its surface and those surfaces of elements attached

to it (subject to the strength of their connections). An example

of this approach is provided by the United Kingdom Regulations [44].

4 . Specifications for Reinforcement and Reinforced Connections : This

460
approach removes from the structural designer the responsibility

for considering successively the application of abnormal loadings

to various portions of the structure. It does so by specifying

reinforcement for elements and for their connection in such a way

as to insure an adequate strength and ductility in the event of

abnormal loadings. An example of this is provided by the CEB

Regulations [45] used in France and in the soon-to-be ratified

Unified Code in the United Kingdom [46].

5. Deemed to Satisfy Clauses : Finally, there have emerged a number

of documents in which it is stated that design in accordance with

certain national standards is deemed to satisfy the intent of

other standards dealing explicitly with the risk of progressive

collapse. Examples of this include BS 449, Design for Steel

Structures, in the United Kingdom [47].

In the United States there have been two cases where criteria have been

developed explicitly to deal with progressive collapse, albeit applied in a limited manner. One of these involved the preparation of the Guide Criteria

[5] for use in the evaluation of the housing systems demonstrated as a part

of the Operation BREAKTHROUGH program. These criteria were prepared by the

National Bureau of Standards on behalf of the Department of Housing and Urban

Development. Expressed in performance language, they were an adaptation of

the Fifth Amendment to the U.K. Building Regulations 1970 [48], and expressed

both the alternate path approach and the use (where absolutely necessary)

of the specified abnormal loading approach (5 psi). These criteria were used

in the evaluation of multistory systems in the BREAKTHROUGH program.

The second set of criteria [6] was prepared by the Federal Housing

Administration of the Department of Housing and Urban Development for use in

the preparation of Structural Engineering Bulletins for those industrialized

systems being designed and built with a FHA mortgage guarantees. Essentially,

FHA based these criteria on the British Standard Code of Practice 116,

Addendum No. 1 for the Design of Large Concrete Panels [44].

There is no shortage of literature discussing the reaction of the

building community to the introduction of criteria against progressive

collapse in high-rise buildings. The reader is referred to Collins [49],

Short and Miles [50], Rodin [51, 52], Creasy [53], Ferahian [54, 55],

Lewicki [56, 57], and a collection of the discussions of structural engineers

at a special meeting on the subject [58, 59]. It is a subject which has

generated a great deal of interest among structural engineers.

IX. Building Response to Abnormal Loadings

This section will deal with the response of buildings to abnormal

loadings from three standpoints: cases of progressive collapse, experimental

studies, and analyses.

A . Cases of Progressive Collapse

A number of people including Griffiths [1], Rodin [52], and Ferahian

[54] have provided considerable insight into the behavior of the Ronan

Point building. Slack [60], has discussed two other instances of explosions

in reinforced concrete buildings, albeit factory buildings. The first case study is concerned with an explosion in a 4-story cast-i n-pl ace framed structure, while the second illustrates the nature of blast damage within a singlestory precast concrete framed building. In the cast-i n-pl ace structure the

progression of the explosion pressure wave is traced and the resulting damage

to the external cladding as well as fire damage are described. The precast

461

493-610 O - 73 - 31
concrete frame building suffered column damage due to the confinement of the

primary explosion. In both of the buildings, large areas of cladding or their connections, were weaker than the main structural frame, resulting in

limited overall damage to the buildings. From the first case, it appears that

the framed cas t- i n

- pi ace structure had a greater inherent resistance to damage

than would be indicated by a straight-forward analysis taking account of

structural continuity. In the precast frame structure, it was found that a precast frame can have a greater resistance to collapse than would seem apparent from a basic reinforced concrete design analysis. However, it is

Slack's recommendation that full or partial continuity at the cast- i n-pl ace joint be provided to give individual members greater resistance to damage.

A short report [61] was given on the bombing of an Army Officer's

building in Aldershot near London in February 1972. The Irish Republican

> Army claimed to have carried out the bombing using 280 pounds of gelignite.

The building did not collapse. The walls of one-half of the building were

blown away completely, but the structural frame of reinforced concrete remained

intact.

A gas explosion destroyed a row of 22 single-story shops in Clarkston,

Glasgow, Scotland. Below the shops, were a series of basement voids into

which gas seeped from a nearby broken main, and concentrated until it was ignited. The explosive pressure has been estimated to be at least 7.3 psi

(and possibly as high as 14.6 psi). The maximum damage occurred about half

way along the row. At the location of the explosion, floor slabs were lifted

and the collapse of the front row of columns was attributed to the failure of

12 in by 18 in reinforced concrete beams and cross beams at ground level.

On the night of March 6, 1972, an explosion occurred in an 11-story

apartment building in Barcelona, Spain, resulting in the collapse of a portion

of the building (See Figure 8) and the deaths of 18 people and many injuries.

It was concluded [8] that the collapse was the result of a conventional explosive. Basically, the building is a load-bearing brick structure. From

evidence available, it would appear that the explosion occurred in an apartment

on the fourth floor, destruction of this unit then leading to the progressive

collapse of all stories above. Falling debris caused destruction of the rooms directly below the apartment. The opinions given in the report [8J are based

upon evidence available at the time of its preparation in July 1972, and may

not be the final word as to the cause.

If it is assumed that the apartment in question were occupied at the

rate of 4 persons per apartment, then the death toll represents 18 out of 44

affected by the incident, 41 percent.

It was reported [62] that a car crashed into a five-story tenament

building in New York dislodging a column and sending tenants and portions of

the structure crashing into the street.

A considerable discussion of cases of progressive collapse in Canada

and the U.S. in the years 1968 to 1972 has been given by Allen and Scriever

[7].

B . Experimental Studies

It is appropriate to discuss these studies from the standpoint of the

materials concerned.

B.l Reinforced Concrete Panels

Ronan Point provided considerable impetus for construction companies

in the United Kingdom to expand their already large experimental efforts and

462
Figure 8 Apartment building at Calle Santa Amelia,

Barcelona, Spain following a progressive collapse

on March 6, 1972.

463
to develop improved procedures for design. Accordingly, the large construction companies marketing concrete panelized systems carried out evaluations

of their systems to determine whether they met the new criteria [2].

Much testing has been carried out in the U.K., by the Building Research

Establishment and Imperial College, concerning the ability of concrete panel

structures to bridge over the loss of structural elements.

Outside the United Kingdom, considerable experimental work has been

carried out by Hanson and Olesen [63] in Denmark. Tests were made to determine the strength and stiffness of verti cal -keyed shear joints between wall

elements of prefabricated concrete. It is known that Olesen is continuing

with a series of laboratory tests, in which are simulated two stories and two

bays of a building that is of precast concrete panel construction. In these

tests, wall and floor panels will be successively removed to determine the

distress in the remaining structure.

Granstrom, in Sweden, has reported [64] model tests, to scale of 1:20,

involving studies of forces in joints and framework deformations of buildings

that have sustained local damage. The tests relate mainly to domestic and

office buildings of precast concrete. The results show that the provision

of joint connections, even of moderate strength, can reduce dramatically the

probability of collapse of buildings. The shear resistance of vertical

joints in large concrete panel construction, and the ability of the joints

to transmit horizontal in-plane tensile forces, has been studied extensively

and the various contributions are too numerous to list in this report. The same is equally true of horizontal connections between floor panels and the

top of wall panels. The reader is referred to the considerable volume of

information presented in March 1970, at a Symposium [65] for Joints in Precast

Concrete Components held in the United Kingdom.

B.2 Load-Bearing Masonry

Wilton, Gabrielsen, Edmunds, and Bechtel [66] reported the early progress

on a long range program with the objective of developing improved methods of

predicting the structural response, failure modes, and debris characteristics

of masonry wall panels. This information is required by the Office of Civil

Defense in order to develop improved shelter systems. The study combines

both analytical and experimental work and, in the period reported, the authors

have developed a statistical failure theory for brick structures, a failure

theory for wall panels, and the development of an analytical program whereby

wall panel failure predictions may be used in the design of an experimental

test program. Their research included testing of brick wall panels, interior

gypsum board wall panels, concrete wall panels and a small number of tests to

investigate the effect of air blast on shelter ventilating equipment. A later report by Wilton, Gabrielsen, and Morris [67] describes the results of

the continued investigation of the response to blast loading of full-scale

wall panels of relatively brittle materials, notably non-reinforced brick.

Information such as element-failure times, energy transmitted to a building

frame, and the influence of support conditions and other geometric factors

were obtained from the tests. Loading tests were carried out on walls which

completely closed a test tunnel, on walls with 17.5 percent doorway openings,

and on walls with 16.7 and 27 percent window openings.

In the United Kingdom, it is now a requirement under the Building

Regulations [48], that structures of five stories and more shall remain

stable after the removal of a specified length of load bearing wall, although

at a substantially reduced safety factor. Sinaj and Hendry [68] describe

three experiments that had the objective of confirming that this could be

achieved in a simple five-story brick cross-wall structure. In each experiment, a section of the main load bearing wall was removed at ground level to

test the stability of the structure in a damage condition, as might occur

464
following an internal explosion. Measurements were made of the applied loads,

deflections and strains. The theoretical conclusion that the structure would

remain stable under these conditions was confirmed and some information was obtained concerning the strength of 114 mm. (4.5 inches) thick wall panels

subjected to lateral loadings.

C . Analysis

The most extensive work, in the area of analysis of building response

to overpressure loading, is that of Newmark [69] who prepared a state-of-theart report presented at the August 1972 Conference on the Planning and Design

of Tall Buildings. In this paper, he discussed the effects on buildings of

external loadings of a transient or impulsive nature where these loadings

can arise from the detonation of explosives, including gas or other sources,

from sonic boom from aircraft, or from accidental impact. Fundamental relations for developing blast resistance design procedures are also presented

based upon the work of Newmark and others over a period of many years.

Other analytical procedures have been reviewed earlier in this paper in

the context of their application.

X. Conclusions

This report is in the nature of a progress document, coming at the end

of the first year of a study of abnormal loadings and progressive collapse.

This study is being carried out by the National Bureau of Standards on behalf

of the Department of Housing and Urban Development.

The report has discussed the state of knowledge with respect to abnormal

loadings on buildings, the response of buildings and building elements to

these loadings, and criteria by which the risk of progressive collapse might

be minimized.

Probably very few buildings, of the type constructed in the past, would

be susceptible to progressive collapse in the event of an abnormal loading.

To date, few incidents of progressive collapse have occurred in the USA, and none approach the magnitude of the Ronan Point incident in the United Kingdom

The patterns of siting, design, and construction of buildings are changing, however, and the frequency of occurrence of certain abnormal loadings on buildings is increasing. The NBS study is concerned not so much with U.S.

buildings of a type constructed in the past, but with those types constructed

now, and in the future. There is reason to believe that, in the absence of new criteria to minimize the risk of progressive collapse, the number of

buildings, that are susceptible, will otherwise increase.

It should not be assumed that conventional systems are free from the

risk of progressive collapse. Rather, studies should be made to determine

the general degree of susceptibility to progressive collapse for multistory

buildings of all construction types. There is also need to study certain

abnormal loadings on buildings for which little knowledge is available at

present, such as unconfined vapor cloud explosions and vehicular impact.

Several other countries have implemented criteria to minimize the risk

of progressive collapse. The data presented and discussed in Chapters 5 and

6 of this report are believed to express a lower bound to the probability of

abnormal loadings in the USA. Yet these data indicate that, for buildings

susceptible to progressive collapse in the USA, the risk substantially

exceeds that which prompted other countries to implement criteria. Furthermore, the risk of fatality appears to be on a par with that for fire.

465
It is concluded, therefore, that U.S. s tandards -wri ti ng bodies should

adopt appropriate rational criteria as soon as possible.

XI. Acknowledgments

Several of the author's colleagues and associates have contributed

valuable advice during the preparation of this report, in particular C. P. Siess, Professor of Civil Engineering, University of Illinois and John E. Breen, Professor of Civil Engineering, University of Texas. Dr. Eric F. P. Burnett, Visiting Researcher to NBS from the University of Waterloo, Canada,

made important contributions to this study. These contributions are gratefully acknowledged.

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Gas Explosions-Report of Further Tests At Potters Marston," Fourth

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Society, London, England.

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26. West, H. W. H., "A Note on the Resistance of Glass Windows to Pressures

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Gas Industry, 1970 Data," Washington, D.C., 1970.

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468
ASCE National Structural Engineering Meeting Preprint 1656, Ohio,

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le Calcul et I'Execution des Constructions en Panneax Assembles de

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Amendment) Regulations," Her Majesties Stationery Office, London,

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for Large Panel Construction," Concrete, Vol. 3, No. 4, p. 121-123,

London, England (April 1969).

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Consulting Engineer, London, England (August 1969).

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Building in London--Its Cause and Consequence," CIB Symposium on Tall

Buildings, Moscow, (12-15 October 1971).

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Regulations," Concrete, Vol. 4, No. 6, p. 216-218, London, (June 1970).

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"National Research Council of Canada, Ottawa, (July 1971).

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469
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Damage. Forces in Element Joint-Model Tests)." Statens Institut for

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Structural Response of Wall Panels," URS Research Company, Technical

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of Wall Panels," URA Research Company, California, (July 1971).

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470
APPENDIX C

LIST OF PARTICIPAIJTS

471
List of Participants

Dr. S. T. Al germi ssen

Di rector

Sei smol ogi cal Research Group

Environmental Research Laboratories

NOAA/ERL/ESL

Boulder, Colorado 80303

Mr. J. Marx Ay res Principal

Ayres , Cohen and Hayakawa

1180 South Beverly Drive

Los Angeles, California 90035

Mr. Paul E. Baseler

Consultant

Route 3, Box 150

House Springs, Missouri 63051

Dr. Roger Borcherdt

Geophysi ci st

U.S. Geological Survey

Department of the Interior

345 Middlefield Road

Menlo Park, California 94025

Dr. Boris Bresler

Professor of Civil Engineering

University of California

777 Davis Hall

Berkeley, California 94720

Dr. Charles G. Culver

Disaster Research Coordinator

Office of Federal Building Technology

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Dr. Neville C. Donovan

Senior Engineer, Partner

Dames and Moore

500 Sansome Street

San Francisco, California 94111

Mr. Mark Fintel

Di rector

Engineering Design Services

Portland Cement Association

33 West Grand Avenue

Skokie, Illinois 60078

Mr. Dale R. Gatlin

Director of Code Research

State of Indiana (NCSBCS)

300 Graphic Arts Building

215 North Senate Avenue

Indianapolis, Indiana 46204

Dr. W. J. Hall

Professor of Civil Engineering

University of Illinois

408 Civil Engineering Building

Urbana, Illinois 61801

Mr. W. A. Heitmann

Chief, Structures Section

Office of the Chief of Engineers

Department of the Army

Washington, D.C. 20314

Mr. Thomas A. Herrin

Director of Community Services and

Regional Development

Coastal Bend Council of Governments

International Airport

Corpus Christi, Texas 78408

Mr. Robert M. Dillon

Executive Director

Building Research Advisory Board

National Academy of Sciences

2101 Constitution Avenue

Washington, D.C. 20418

Mr. John Dirst

Staff Engineer

Defense Civil Preparedness Agency

The Pentagon

Washington, D.C. 20301

Dr. George Housner

Professor of Civil Engineering

California Institute of Technology

Pasadena, California 91109

Mr. Arnold R. Hull

Associate Director for Climatology

NOAA-EDS

3300 Whiteheaven Street, N.W.

Washington, D.C. 20235

472
Mr. Jack R. Janney

Partner

Wiss, Janney, Elstner

330 Pfingsten Road

Northbrook, Illinois 60062

Mr. Samuel Kramer

Chief

Office of Federal Building Technology

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Dr. Howard Kunreuther

Professor of Economics

Wharton School of Finance & Commerce

University of Pennsylvania

Philadelphia, Pennsylvania 19104

Mr . Andrew Lai ly

Assistant Chief Engineer

American Institute of Steel

Constructi on

101 Park Avenue

New York, New York 10017

Mr. James Lefter

Di rector

Civil Engineering Service

Veterans Administration (085)

811 Vermont Avenue, N.W.

Washington, D.C. 20420

Dr. H. S. Lew

Structural Research Engineer

Structures, Materials, and Life

Safety Division

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Mr. John C. Loevenguth

Vice President

Hillman, Biddison, Loevenguth

1730 West Olympia Boulevard

Los Angeles, California 90015

Dr. R. D. Marshall

Structural Research Engineer

Structures, Materials, and Life

Safety Division

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Dr. R. B. Matthiesen

Di rector

Seismological Field Survey

National Oceanic and Atmospheric

Admi ni strati on Department of Commerce

890 Main Street

San Francisco, California 94105

Mr. Frank E. McClure

Consulting Structural Engineer

McClure and Messinger

366 40th Street

Oakland, California 94609

Mr . Ugo Morel 1

i

Program Officer

Office of Emergency Preparedness

(Room 316)

605 17th Street, N.W.

Washington, D.C. 20504

Dr. Joseph Penzien

Professor of Structural Engineering

University of California

Berkeley, California 94720

Mr. C. W. Pinkham

Pres i dent

S. B. Barnes and Associates

2236 Beverly Boulevard

Los Angeles, California 90057

Dr. John B. Scalzi

Office of Research and Technology

Department of Housing and Urban

Devel opment

451 7th Street, S.W. (Room 4114)

Washington, D.C. 20410

Mr. James Lord

Director of Systems Engineering

Albert C. Martin & Associates

Post Office Box 60147

Los Angeles, California 90060

Mr . Rol and L . Sharpe

Executive Vice President

John A. Blume and Associates

Sheraton Palace Hotel

130 Jessie Street

San Francisco, California 94105

473
Mr. James E. Slosson

Supervisory Engineer/Geologist

James E. Slosson and Associates

15373 Valley Vista Boulevard

Sherman Oaks, California 91403

Dr. Norman F. Somes

Chief, Structures Section

Structures, Materials, and Life

Safety Division

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Mr. Tseng-Yao Sun

Vice President

Ayres, Cohen and Hayakawa

1180 South Beverly Drive

Los Angeles, California 90035

Dr. Charles C. Thiel

Program Manager

Division of Advanced Technology

Applications

National Science Foundation

1800 G Street, N.W.

Washington, D.C. 20550

Mr. Christopher G. Tyson

Chi ef Civil Engi neer

Dade County Public Works

1351 N.W. 12th Street

Mi ami , Florida 33125

Dr. Joseph Vel lozzi

Structural Engineer

Ammann and Whitney

1 1 1 Eighth Avenue

New York, New York 10011

Mr. Seymour Wengrovitz

Staff Director

Advisory Services Division

Defense Civil Preparedness Agency

The Pentagon

Washington, D.C. 20301

Dr. Gilbert F. White

Professor of Geography

Institute of Behavioral Science

University of Colorado

Boulder, Colorado 80302

Dr. Robert Whitman

Professor of Civil Engineering

Massachusetts Institute of

Technol ogy

Cambridge, Massachusetts 02139

Dr. John H. Wiggins, Jr.

Pres i dent

J. H. Wiggins Company

2516 Via Tejon

Palos Verdes Estates, California

90274

Dr. James R. Wright

Di rector

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

Dr. Richard N. Wright

Deputy Director - Technical

Center for Building Technology

National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

474
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET NBS-R,SS-46

3, Recipient's Accession No.

4. TITLE AND SUBTITLE Building Practices For Disaster Mitigation. Proceedings Of A Workshop held at the National Bureau of Standards, Boulder,

Colorado, August 28-September 1, 1972

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S) R.N. Wright, S. Kramer, C. Culver

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. fitoject/Task/Work Unit No. A6O55OO

11. Contract/Grant No.

12. Sponsoring Organization Name and Address National Science Foundation, Research Applied to National

Bureau of Standards, Washington, D.C. 20234

13. Type of Report & Period Covered m 11 /r/1 Q In ^ j.nx>erini, i i / il—^i i il 14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here.) The National workshop on building practices for disaster mitigation was concerned

with earthquakes, extreme winds, and similar dynamic hazards. These proceedings

present recommendations derived at the workshop and addressed to policy makers in

government and industry, as well as practitioners in engineering, architecture,

land use planning, and the earth and meteorological sciences. The recommendations

evaluate current building practices, define opportunities for improving current

practice from documented research findings, and recommend research to fill gaps

in knowledge. Recommendations are made for implementation of improved practices

at professional and policy levels. The objectives include avoidance of human

suffering, reduction of property loss, and maintenance of vital function in

buildings under conditions threatening disaster. Fifteen review articles were prepared by experts in the professions and research disciplines to define the state of the art in disaster mitigation and to guide discussions at the woricshop; the

articles are included in the proceedings.

17. KEY WORDS (Alphabetical order, separated by semicolons) Building; earthquakes; hazards; land use; structural engineering; wind effects.

18. AVAILABILITY STATEMENT

UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

483

22. Price $5. 30

USCOMM-OC 66244.P71

U. S. GOVERNMENT PRINTING OFFICE : 1973 O - 493-610


U.S. DEPARTMENT OF COMMERCE

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Washington, D.C. 20234

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